RDMA/siw: Fix SQ/RQ drain logic

Storage ULPs (e.g. iSER & NVMeOF) use ib_drain_qp() to drain
QP/CQ. Current SIW's own drain routines do not properly wait until all
SQ/RQ elements are completed and reaped from the CQ. This may cause touch
after free issues.  New logic relies on generic
__ib_drain_sq()/__ib_drain_rq() posting a final work request, which SIW
immediately flushes to CQ.

Fixes: 303ae1cd

 ("rdma/siw: application interface")
Link: https://lore.kernel.org/r/20191004125356.20673-1-bmt@zurich.ibm.com
Signed-off-by: Krishnamraju Eraparaju <krishna2@chelsio.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>