ext2: introduce helper for xattr header validation

Introduce helper function ext2_xattr_header_valid()
for xattr header validation and clean up the header
check ralated code.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Chengguang Xu <cgxu519@zoho.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>