fat: reject BPB volumes whose data area starts beyond total sectors

fat: reject BPB volumes whose data area starts beyond total sectors

fat_fill_super() subtracts sbi->data_start from the BPB total sector count
before computing the number of clusters.  A malformed image can declare a
total sector count smaller than data_start, causing the subtraction to
underflow and the mount code to derive a plausible cluster count from the
FAT length instead.

Reject such images before the subtraction.  In QEMU, a crafted FAT image
with total_sectors=2 and data_start=3 mounted successfully before the fix
and reading a file returned bytes stored past the BPB-declared end of the
volume.  With this change, the same image is rejected during mount.

Assisted-by: Codex:gpt-5.5-cyber-preview
Link: https://lore.kernel.org/20260605155216.2126545-1-sam.moelius@trailofbits.com
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>