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/fs/fs.c

@@ -43,6 +43,7 @@
 #include "buf.h"
 #include "fsstat.h"
 #include "file.h"
+#include "fat.h"
 
 extern uint uptime(void);
 
@@ -277,6 +278,7 @@ static const struct fs_ops v1_ops = {
 
 // Defined in fs/schfs/vfs.c, which is only linked when SchFS is built in.
 extern const struct fs_ops schfs_ops;
+extern const struct fs_ops fat_ops;
 // Defined in fs/procfs.c -- the synthetic /proc.
 extern const struct fs_ops procfs_ops;
 extern const struct fs_ops devfs_ops;
@@ -290,7 +292,7 @@ extern const struct fs_ops devfs_ops;
 int
 fs_readonly(uint dev)
 {
-  return SCHFS_IS_SNAP_DEV(dev);
+  return SCHFS_IS_SNAP_DEV(dev) || (FAT_IS_DEV(dev) && fat_readonly(dev));
 }
 
 static const struct fs_ops *
@@ -302,6 +304,7 @@ ops_for(uint dev)
   if (SCHFS_IS_ANY_DEV(dev)) return &schfs_ops;
   if (dev == PROCFS_DEV) return &procfs_ops;
   if (dev == DEVFS_DEV)  return &devfs_ops;
+  if (FAT_IS_DEV(dev))   return &fat_ops;
   return &v1_ops;
 }
 
@@ -468,6 +471,38 @@ iget(uint dev, uint64 inum)
   return ip;
 }
 
+// Is (dev, inum) in the inode cache with a reference? For a filesystem that
+// keeps its own table of inode numbers and wants to know which it can drop.
+int
+iheld(uint dev, uint64 inum)
+{
+  int held = 0;
+  acquire(&itable.lock);
+  for (struct inode *ip = &itable.inode[0]; ip < &itable.inode[NINODE]; ip++)
+    if (ip->ref > 0 && ip->dev == dev && ip->inum == inum) {
+      held = 1;
+      break;
+    }
+  release(&itable.lock);
+  return held;
+}
+
+// Is anything on dev held, other than `except` (a mounted root, which the
+// mount itself holds)? For umount.
+int
+ibusy(uint dev, uint64 except)
+{
+  int busy = 0;
+  acquire(&itable.lock);
+  for (struct inode *ip = &itable.inode[0]; ip < &itable.inode[NINODE]; ip++)
+    if (ip->ref > 0 && ip->dev == dev && ip->inum != except) {
+      busy = 1;
+      break;
+    }
+  release(&itable.lock);
+  return busy;
+}
+
 struct inode *
 idup(struct inode *ip)
 {