RDMA: Update missed conversion of flush_scheduled_work()

Commit f0626710

 ("RDMA: Update workqueue usage") introduced ib_wq
and removed the use of flush_scheduled_work(); however, during the merge
process one chunk was lost in ib_sa_remove_one().  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>