RDMA: Check umem pointer validity prior to release

Update ib_umem_release() to behave similarly to kfree() and allow
submitting NULL pointer as safe input to this function.

Fixes: a52c8e2469 ("RDMA: Clean destroy CQ in drivers do not return errors")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/i40iw/i40iw_verbs.c

@@ -2004,8 +2004,7 @@ static int i40iw_dereg_mr(struct ib_mr *ib_mr, struct ib_udata *udata)
 	struct cqp_commands_info *cqp_info;
 	u32 stag_idx;
 
-	if (iwmr->region)
-		ib_umem_release(iwmr->region);
+	ib_umem_release(iwmr->region);
 
 	if (iwmr->type != IW_MEMREG_TYPE_MEM) {
 		/* region is released. only test for userness. */