librdmacm: Abstract ibverbs SRQ creation

librdmacm: Abstract ibverbs SRQ creation

Support QPs with SRQs.  If a user allocates an SRQ on an
rdma_cm_id, we post receive messages directly to the SRQ.
This also allows us to handle XRC SRQs, which may be associated
with an rdma_cm_id, but without a corresponding QP.

To handle registering memory, we store the PD associated
with an rdma_cm_id directly with the id, rather than finding
the PD using a QP pointer.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>