IB/cm: Suppress gcc 7 fall-through complaints

Avoid that gcc 7 reports the following warning when building with W=1:

warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/cm.c

@@ -2808,6 +2808,7 @@ int ib_send_cm_mra(struct ib_cm_id *cm_id,
 			msg_response = CM_MSG_RESPONSE_OTHER;
 			break;
 		}
+		/* fall through */
 	default:
 		ret = -EINVAL;
 		goto error1;