fat: FAT12, FAT16 and FAT32 with long names, read and write

The filesystem every other system can read: a USB stick, an SD card, the
EFI system partition. It is here first for the hardware report, which has
to leave a machine on something that can be read on another one.

  mount -t vfat /dev/usb0p1 /mnt        (msdos and fat work too; -o ro)

Written from Microsoft's specification, and matched to Linux vfat where
the specification leaves a choice: 8.3 names with the NT lowercase bits,
long-name entries otherwise, Linux's alias rule (REPORTS, then BIGNUM~1),
case-insensitive lookup, UTC times, the boot-sector dirty bit, every FAT
copy and FSInfo kept up to date. It uses its own sector cache over the
new blkdev_xfer, since a partition need not be a whole number of 4 KB
blocks, and every operation that changes something reaches the disk
before it returns. Inode numbers come from the entry position, or the
first cluster for a directory, plus a small table for entries that moved.

Around it:
- /dev lists every disk and partition the block layer found (major 8,
  minor = block device), and raw reads and writes on them go by sector.
- The ESP and MBR FAT partitions are registered, typed so the root probe
  passes over them. MBR entries with a bad status byte are skipped, so a
  superfloppy's boot code is not read as a partition table.
- statfs, sync, fsync, umount (EBUSY while in use), shutdown, reboot and
  the power button all know about FAT.
- create undoes and reports a failed ialloc or dirlink (ENOSPC, EINVAL,
  EPERM) instead of panicking; fs_ops gains refuse_types and no_hardlinks.
- filewrite returns a short write instead of panicking on it.
- utimensat is implemented. As a stub returning 0 it made toybox touch
  believe the file existed, so `touch newfile` created nothing on any
  filesystem.

Tested in QEMU against mkfs.vfat images (FAT12 floppy, FAT16 on MBR,
FAT32 superfloppy and GPT ESP) by writing, renaming, deleting, filling to
ENOSPC and truncating there, then checking with fsck.fat -n and mtools on
the host: all clean, contents match.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/dev/disk.c

@@ -30,6 +30,7 @@
 #include "buf.h"
 #include "file.h"
 #include "errno.h"
+#include "blkdev.h"
 
 // The byte offset is 64-bit, all the way from struct file's off through the
 // devsw signature to bread's block number. It used to narrow to a 32-bit uint
@@ -38,11 +39,65 @@
 // not a refusal at 4 GB but silent wraparound past it. Nothing here may narrow
 // it again: an NVMe namespace or any modern disk is far beyond that.
 
+// A disk or partition that registered with the block layer (minor 4 onward):
+// a SATA or NVMe disk, a USB stick, any partition on them. Moved through
+// blkdev_xfer in whole sectors rather than through the buffer cache, because a
+// partition need not be a whole number of BSIZE blocks and the buffer cache's
+// last block of one would run into the next. A read at the end returns 0, as
+// a file's does, and a write there fails with ENOSPC.
+static int
+blk_rw(uint dev, uint64 off, char *p, int n, int write)
+{
+  uint32 ss = 0;
+  uint64 nsec = blkdev_sectors(dev, &ss);
+  if (nsec == 0)
+    return -ENXIO;
+  if (n <= 0)
+    return 0;
+  uint64 size = nsec * ss;
+  if (off >= size)
+    return write ? -ENOSPC : 0;
+  if ((uint64)n > size - off)
+    n = (int)(size - off);
+
+  char *tmp = kalloc();
+  if (!tmp)
+    return -ENOMEM;
+  int done = 0;
+  while (done < n) {
+    uint64 pos = off + (uint64)done;
+    uint64 lba = pos / ss;
+    uint   in  = (uint)(pos % ss);
+    uint32 cnt = (uint32)(BSIZE / ss);
+    if (lba + cnt > nsec)
+      cnt = (uint32)(nsec - lba);
+    int want = (int)(cnt * ss) - (int)in;
+    if (want > n - done)
+      want = n - done;
+    // Whole sectors are written as they are; a partial one is read first.
+    uint32 rcnt = (in + want + ss - 1) / ss;
+    if (!write || in || want % ss)
+      if (blkdev_xfer(dev, lba, rcnt, tmp, 0) < 0)
+        break;
+    if (write) {
+      memmove(tmp + in, p + done, want);
+      if (blkdev_xfer(dev, lba, rcnt, tmp, 1) < 0)
+        break;
+    } else
+      memmove(p + done, tmp + in, want);
+    done += want;
+  }
+  kfree(tmp);
+  return done ? done : -EIO;
+}
+
 int
 diskread(struct inode *ip, uint64 off, char *dst, int n)
 {
   int dev = ip->minor;
 
+  if (dev >= BLKDEV_FIRST)
+    return blk_rw((uint)dev, off, dst, n, 0);
   if (!ide_disk_present(dev))
     return -ENXIO;
   if (n <= 0)
@@ -71,6 +126,8 @@ diskwrite(struct inode *ip, uint64 off, char *src, int n)
 {
   int dev = ip->minor;
 
+  if (dev >= BLKDEV_FIRST)
+    return blk_rw((uint)dev, off, src, n, 1);
   if (!ide_disk_present(dev))
     return -ENXIO;
   if (n <= 0)