RDMA/core: Export ib_open_qp() to share XRC TGT QPs

XRC TGT QPs are shared resources among multiple processes.  Since the
creating process may exit, allow other processes which share the same
XRC domain to open an existing QP.  This allows us to transfer
ownership of an XRC TGT QP to another process.

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