Revert "IPoIB: Make ipoib_mcast_stop_thread flush the workqueue"

This reverts commit bb42a6dd02fb2901a69dbec2358810735b14b186.

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>

diff --cc drivers/infiniband/ulp/ipoib/ipoib.h
Simple merge

diff --cc drivers/infiniband/ulp/ipoib/ipoib_ib.c
Simple merge

diff --cc drivers/infiniband/ulp/ipoib/ipoib_multicast.c
Simple merge