GFS2: WQ_NON_REENTRANT is meaningless and going away

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: Tejun Heo <tj@kernel.org>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: cluster-devel@redhat.com