RDMA/cma: Set qkey for AF_IB

Allow the user to specify the qkey when using AF_IB.  The qkey is
added to struct rdma_ucm_conn_param in place of a reserved field, but
for backwards compatability, is only accessed if the associated
rdma_cm_id is using AF_IB.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>