nfsd: Avoid some useless tests

memdup_user() can't return NULL, so there is no point for checking for it.

Simplify some tests accordingly.

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>