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>