ext4: correct comment of h_checksum

Checksum of xattr block is always crc32c(uuid+blknum+xattrblock), see
ext4_xattr_block_csum_set for detail. Remove incorrect comment that
"id = inum if refcount=1".

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://patch.msgid.link/20240606125508.1459893-4-shikemeng@huaweicloud.com


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