IB/ipoib: Use cancel_delayed_work_sync for neigh-clean task

The neigh_reap_task is self restarting, but so long as we call
cancel_delayed_work_sync() it will be guaranteed to not be running and
never start again. Thus we don't need to have the racy
IPOIB_STOP_NEIGH_GC bit, or the confusing mismatch of places sometimes
calling flush_workqueue after the cancel.

This fixes a situation where the GC work could have been left running
in some rare situations.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>