gfs2: Cease delete work during unmount

Add a check to delete_work_func() so that it quits when it finds that
the filesystem is deactivating.  This speeds up the delete workqueue
draining in gfs2_kill_sb().

In addition, make sure that iopen_go_callback() won't queue any new
delete work while the filesystem is deactivating.

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