ext4: correct comment of ext4_xattr_cmp

The ext4_xattr_cmp never returns negative error number. Correct possible
return value in ext4_xattr_cmp's comment.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://patch.msgid.link/20240606125508.1459893-2-shikemeng@huaweicloud.com


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