IPoIB: Turn on interface's carrier after broadcast group is joined

Do netif_carrier_on() right after the IPv4 broadcast multicast group
is joined, rather than waiting for all of the initial set of multicast
group joins to finish.  This allows at least IPv4 traffic to limp
along on broken fabrics where not all multicast groups can be joined.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>