ipoib_mcast_detach() does nothing except call ib_detach_mcast(), so just
use the core API in the one place that does a multicast group detach.

add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-105 (-105)
function                                     old     new   delta
ipoib_mcast_leave                            357     319     -38
ipoib_mcast_detach                            67       -     -67
Signed-off-by: Roland Dreier <rolandd@cisco.com>