RDMA/ucma: Add ability to query GID addresses

Part of address resolution is mapping IP addresses to IB GIDs.  With
the changes to support querying larger addresses and more path records,
also provide a way to query IB GIDs after resolution completes.

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