nfsd: Propagate some error code returned by memdup_user()

Propagate the error code returned by memdup_user() instead of a hard coded
-EFAULT.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>