xprtrdma: checking for NULL instead of IS_ERR()

The rpcrdma_create_req() function returns error pointers or success.  It
never returns NULL.

Fixes: f531a5db ('xprtrdma: Pre-allocate backward rpc_rqst and send/receive buffers')
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/backchannel.c

@@ -42,8 +42,8 @@ static int rpcrdma_bc_setup_rqst(struct rpcrdma_xprt *r_xprt,
 	size_t size;
 
 	req = rpcrdma_create_req(r_xprt);
-	if (!req)
-		return -ENOMEM;
+	if (IS_ERR(req))
+		return PTR_ERR(req);
 	req->rl_backchannel = true;
 
 	size = RPCRDMA_INLINE_WRITE_THRESHOLD(rqst);