rdma/uverbs: Export XRC TGT QPs to user space

rdma/uverbs: Export XRC TGT QPs to user space

Allow user space to operate on XRC TGT QPs the same as other
types of QPs, with one notable exception.  Since XRC TGT QPs
may be shared among multiple processes, the XRC TGT QP is
allowed to exist beyond the lifetime of the creating process.

The process that creates the qp is allowed to destroy it, but
if the process exits without destroying the qp, then the qp
will be left bound to the lifetime of the XRCD.

TGT QPs are not associated with CQs or a PD.

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