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>
Signed-off-by: Roland Dreier <roland@purestorage.com>