gfs2: Sanitize kthread stopping

Immediately stop the logd and quotad kernel threads when a filesystem
withdraw is detected: those threads aren't doing anything useful after a
withdraw.  (Depends on the extra logd and quotad task struct references
held since commit 7a109f383fa3 ("gfs2: Fix asynchronous thread
destruction").)

In addition, check for kthread_should_stop() in the wait condition in
gfs2_quotad() to stop immediately when kthread_stop() is called.

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