Revert "IPoIB: fix mcast_dev_flush/mcast_restart_task race"

This reverts commit e5d1dcf1.

The series of IPoIB bug fixes that went into 3.19-rc1 introduce
regressions, and after trying to sort things out, we decided to revert
to 3.18's IPoIB driver and get things right for 3.20.

Signed-off-by: Roland Dreier <roland@purestorage.com>