dbf2576e ("workqueue: make all workqueues non-reentrant") made
WQ_NON_REENTRANT no-op and the flag is going away.  Remove its usages.

This patch doesn't introduce any behavior changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NSage Weil <sage@inktank.com>
Cc: ceph-devel@vger.kernel.org