RDMA/rdmavt: 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: NBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: NDennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>

drivers/infiniband/sw/rdmavt/qp.c

@@ -807,6 +807,7 @@ struct ib_qp *rvt_create_qp(struct ib_pd *ibpd,
 		if (init_attr->port_num == 0 ||
 		    init_attr->port_num > ibpd->device->phys_port_cnt)
 			return ERR_PTR(-EINVAL);
+		/* fall through */
 	case IB_QPT_UC:
 	case IB_QPT_RC:
 	case IB_QPT_UD: