RDMA/cma: Use rate from IPoIB broadcast when joining IPoIB multicast groups

When joining an IPoIB multicast group, use the same rate as in the
broadcast group.  Otherwise, if the RDMA CM creates this group before
IPoIB does, it might get a different rate.  This will cause IPoIB to
fail joining to the same group later on, because IPoIB uses strict
rate selection.

Signed-off-by: Yossi Etigin <yosefe@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>