gfs2: Don't sleep during glock hash walk

In flush_delete_work, instead of flushing each individual pending
delayed work item, cancel and re-queue them for immediate execution.
The waiting isn't needed here because we're already waiting for all
queued work items to complete in gfs2_flush_delete_work.  This makes the
code more efficient, but more importantly, it avoids sleeping during a
rhashtable walk, inside rcu_read_lock().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>