IB/core: Don't drain non-existent rq queue-pair

The drain_rq function expects a normal receive qp to drain.  A qp can
only have either a normal rq or an srq.  If there is an srq, there
is no rq to drain.  Until the API supports draining SRQs, simply
skip draining the rq when the qp has an srq attached.

Fixes: 765d6774 ("IB: new common API for draining queues")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>