IB/mad: Don't discard BMA responses in kernel

This fixes the problem of incoming BMA responses being dropped due to
a bad "is response" check.  Fix the test to use the ib_response_mad()
predicate, which correctly handles BMA MADs.

This fixes <https://bugs.openfabrics.org/show_bug.cgi?id=988

>.

Signed-off-by: Michael Brooks <michael.brooks@qlogic.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>