rdma/verbs: Cleanup XRC TGT QPs when destroying XRCD

rdma/verbs: Cleanup XRC TGT QPs when destroying XRCD

XRC TGT QPs are intended to be shared among multiple users and
processes.  Allow the destruction of an XRC TGT QP to be done
explicitly through ib_destroy_qp() or when the XRCD is
destroyed.

To support destroying an XRC TGT QP, we need to track TGT QPs
with the XRCD.  When the XRCD is destroyed, all tracked XRC
TGT QPs are also cleaned up.

To avoid stale reference issues, if a user is holding a
reference on a TGT QP, we increment a reference count on
the QP.  The user releases the reference by calling
ib_release_qp.  This releases any access to the QP from
a user above verbs, but allows the QP to continue to
exist until destroyed by the xrcd.

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