IB/ipoib: Use cancel_delayed_work_sync when needed

The work mcast_task can re-queue itself, so instead of doing
cancel && flush_workqueue, that still can leave a queued task
on the air, use cancel_delayed_work_sync.

Also, no need to use lock over the cancel, the original lock was
due to bit assignment setting (IPOIB_MCAST_RUN) that is not in use
anymore.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>