nfsd: handle failure to collect pre/post-op attrs more sanely

Collecting pre_op_attrs can fail, in which case it's probably best to
fail the whole operation.

Change fh_fill_pre_attrs and fh_fill_both_attrs to return __be32, and
have the callers check the return code and abort the operation if it's
not nfs_ok.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>