ext4: correct comment of ext4_xattr_block_cache_insert

There is no return value from ext4_xattr_block_cache_insert, just correct
it's comment about return value.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://patch.msgid.link/20240606125508.1459893-3-shikemeng@huaweicloud.com


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