ext2: introduce helper for xattr entry validation

Introduce helper function ext2_xattr_entry_valid()
for xattr entry validation and clean up the entry
check related 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>