IB/mad: Test ib_create_send_mad() return with IS_ERR(), not == NULL

In case of error, the function ib_create_send_mad() returns an ERR
pointer, but never returns a NULL pointer.  So testing the return
value for error should be done with IS_ERR, not by comparing with
NULL.

A simplified version of the semantic patch that makes this change is
as follows:

(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@correct_null_test@
expression x,E;
statement S1, S2;
@@
x = ib_create_send_mad(...)
<... when != x = E
if (
(
- x@p2 != NULL
+ ! IS_ERR ( x )
|
- x@p2 == NULL
+ IS_ERR( x )
)
 )
S1
else S2
...>
? x = E;
// </smpl>

Signed-off-by: Julien Brunel <brunel@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/core/mad_rmpp.c

@@ -133,7 +133,7 @@ static void ack_recv(struct mad_rmpp_recv *rmpp_recv,
 	msg = ib_create_send_mad(&rmpp_recv->agent->agent, recv_wc->wc->src_qp,
 				 recv_wc->wc->pkey_index, 1, hdr_len,
 				 0, GFP_KERNEL);
-	if (!msg)
+	if (IS_ERR(msg))
 		return;
 
 	format_ack(msg, (struct ib_rmpp_mad *) recv_wc->recv_buf.mad, rmpp_recv);