ext4: verify fast symlink length

Verify fast symlink length stored in inode->i_size matches the string
stored in the inode to avoid surprises from corrupted filesystems.

Reported-by:  <syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com>
Tested-by:  <syzbot+48a99e426f29859818c0@syzkaller.appspotmail.com>
Fixes: bae80473 ("ext4: use inode_set_cached_link()")
Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://patch.msgid.link/20250206094454.20522-2-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>