IB: new common API for draining queues

Add provider-specific drain_sq/drain_rq functions for providers needing
special drain logic.

Add static functions __ib_drain_sq() and __ib_drain_rq() which post noop
WRs to the SQ or RQ and block until their completions are processed.
This ensures the applications completions for work requests posted prior
to the drain work request have all been processed.

Add API functions ib_drain_sq(), ib_drain_rq(), and ib_drain_qp().

For the drain logic to work, the caller must:

ensure there is room in the CQ(s) and QP for the drain work request
and completion.

allocate the CQ using ib_alloc_cq() and the CQ poll context cannot be
IB_POLL_DIRECT.

ensure that there are no other contexts that are posting WRs concurrently.
Otherwise the drain is not guaranteed.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>