RDMA/cxgb4: Protect from possible dereference

Smatch tool reports the following error:
  drivers/infiniband/hw/cxgb4/qp.c:1886
	c4iw_create_qp() error: we previously assumed 'ucontext'
	could be null (see line 1804)

Cc: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/cxgb4/qp.c

@@ -1843,7 +1843,7 @@ struct ib_qp *c4iw_create_qp(struct ib_pd *pd, struct ib_qp_init_attr *attrs,
 	if (ret)
 		goto err_destroy_qp;
 
-	if (udata) {
+	if (udata && ucontext) {
 		sq_key_mm = kmalloc(sizeof(*sq_key_mm), GFP_KERNEL);
 		if (!sq_key_mm) {
 			ret = -ENOMEM;