exfat: fix potential use-after-free in exfat_find_dir_entry()

exfat: fix potential use-after-free in exfat_find_dir_entry()

In exfat_find_dir_entry(), the buffer_head obtained from
exfat_get_dentry() is released with brelse(bh) before the fall-through
TYPE_EXTEND branch reads the directory entry through ep (which points
into bh->b_data):

	brelse(bh);
	if (entry_type == TYPE_EXTEND) {
		...
		len = exfat_extract_uni_name(ep, entry_uniname);
		...
	}

After brelse() drops our reference, nothing guarantees that the
underlying page backing bh->b_data remains valid for the subsequent
exfat_extract_uni_name() read. This is the same pattern fixed in
commit fc961522ddbd ("exfat: Fix potential use after free in
exfat_load_upcase_table()").

Move brelse(bh) so it runs after ep is no longer dereferenced on
each branch.

Confirmed on QEMU x86_64 with CONFIG_KASAN=y + CONFIG_DEBUG_PAGEALLOC=y
+ CONFIG_PAGE_POISONING=y on linux-next, using a crafted exFAT image
(long filename with same-hash collisions forcing the TYPE_EXTEND path).
With a debug-only invalidate_bdev() inserted between brelse(bh) and
the ep read to make the stale-deref window deterministic, the
unpatched kernel faults:

  BUG: KASAN: use-after-free in exfat_find_dir_entry+0x133b/0x15a0
  BUG: unable to handle page fault for address: ffff88801a5fa0c2
  Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
  RIP: 0010:exfat_find_dir_entry+0x1188/0x15a0

With this patch applied, the same instrumented harness completes
cleanly under the same sanitizer stack. I have not reproduced a
crash on an uninstrumented kernel under ordinary reclaim; the
instrumented A/B establishes the lifetime violation and that the
patch closes it, not an unaided triggerability claim.

Fixes: ca06197382bd ("exfat: add directory operations")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>