schfs: read a whole block per tree descent, not half of one

BOUNCE was 2048 while the comment directly above it said "one page at a
time", and the buffer it sizes is a single kalloc() -- so half of every
bounce buffer went unused and every read did twice the work it needed to.

The chunk size is not a copy-size tuning knob. Each trip through the loop
takes the volume sleeplock and builds a fresh B-tree cursor to find the
extent for that offset, so it really sets how many tree descents happen
per byte read. An 834 KB file took 417 of them; it now takes 209.

Measured in the guest, bracketed inside one command line so nothing
depends on host timing: reading 834 KB goes from 1.73s to 0.90s under
TCG, cold and warm alike. Warm equalling cold is itself the tell -- the
cost was never the disk, it was the per-chunk work, which is why KVM was
6.4x faster on the same read while caching made no difference at all.

This does not fix `ls`, and the numbers say why: 2.80s -> 2.65s cold.
loaduvm already reads a page at a time, so exec was never the bottleneck
there. At 241 entries in 2.65s that is ~11ms per file, spent in the
per-entry lookup path rather than in reading anything.

schfs-check passes; bootcheck 108 [ok].

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kernel/fs/schfs/vfs.c

@@ -142,7 +142,18 @@ schfs_vfs_ifree(struct inode *ip)
 // the tree while a copyout could fault. One page at a time keeps that buffer
 // off the 4096-byte kernel stack -- it is kalloc'd -- and bounds how long the
 // volume lock is held.
-#define BOUNCE 2048
+//
+// One filesystem block, which is also one kalloc page: this said 2048 while
+// the comment above it said one page, so half of every bounce buffer went
+// unused and every read did twice the work it needed to.
+//
+// The chunk size is not a copy-size tuning knob. Each trip through the loop
+// takes the volume sleeplock and builds a fresh B-tree cursor to find the
+// extent for that offset, so this is really "how many tree descents per byte
+// read". Measured on an 834 KB file: 417 descents at 2048, 209 at 4096.
+//
+// It must not exceed a page, because the buffer is a single kalloc().
+#define BOUNCE SCHFS_BLKSIZE
 
 static int
 schfs_vfs_read(struct inode *ip, int user_dst, uint64 dst, uint64 off, uint n)