IB/ipoib: Clean up send-only multicast joins

Even though we don't expect the group to be created by the SM we
sill need to provide all the parameters to force the SM to validate
they are correct.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>