RDMA/ucma: Allow user space to specify AF_IB when joining multicast

Allow user space applications to join multicast groups using MGIDs
directly.  MGIDs may be passed using AF_IB addresses.  Since the
current multicast join command only supports addresses as large as
sockaddr_in6, define a new structure for joining addresses specified
using sockaddr_ib.

Since AF_IB allows the user to specify the qkey when resolving a
remote UD QP address, when joining the multicast group use the qkey
value, if one has been assigned.

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