IB/core: Protect against self-requeue of a cq work item

We need to make sure that the cq work item does not
run when we are destroying the cq. Unlike flush_work,
cancel_work_sync protects against self-requeue of the
work item (which we can do in ib_cq_poll_work).

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com&gt;-->
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>