ext4: stop retrying saturated xattr cache entries

ext4_xattr_block_set() retries when a cache entry selected for reuse
has a saturated reference count after taking the buffer lock. The retry
returns to the mbcache lookup without making that entry ineligible, so
it can select the same unusable entry indefinitely. A task spinning
there can hold the parent directory's i_rwsem and leave concurrent
rmdir callers blocked.

Normally a reusable entry has a reference count below
EXT4_XATTR_REFCOUNT_MAX because the count and MBE_REUSABLE_B are
updated under the same buffer lock. A corrupted filesystem can violate
that invariant. The syzbot reproducer reports allocator and xattr
corruption before triggering this retry loop.

Check the untrusted on-disk count before incrementing it, avoiding
overflow, and clear MBE_REUSABLE_B when it is already saturated. The
next lookup then skips the entry that was just proven unusable. This
mirrors the normal transition at EXT4_XATTR_REFCOUNT_MAX; the release
path marks the entry reusable again on the exact 1024-to-1023
transition.

Using the same QEMU harness and guest parameters, current unpatched
Linux hung in 6 of 8 420-second trials with the do_rmdir signature;
representative NMI backtraces caught the owner spinning in
ext4_xattr_block_set(). The patched kernel completed 28 of 28 trials
without a hung-task report; the final twelve trials exercised the
reviewed overflow-safe form of the change. syzbot's patch testing also
completed without reproducing the hang.

Reported-and-tested-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e68dbebd9617a9250e8d
Fixes: 65f8b80053a1 ("ext4: fix race when reusing xattr blocks")
Cc: stable@vger.kernel.org
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Tested-by: syzbot+e68dbebd9617a9250e8d@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20260802065941.1726052-1-matthias.goergens@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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