RDMA/cma: Fix AF_INET6 support in multicast joining

If joining to an AF_INET6 address, we need to map the address to a MGID
in the same way as the IP stack.  The old code would just fall through to
the IPv4 case and generate garbage.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>