elv_unregister: fix possible crash on module unload

An exiting task or process which didn't do I/O yet have no io context,
elv_unregister() should check it is not NULL.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

block/elevator.c

@@ -766,7 +766,8 @@ void elv_unregister(struct elevator_type *e)
 		read_lock(&tasklist_lock);
 		do_each_thread(g, p) {
 			task_lock(p);
-			e->ops.trim(p->io_context);
+			if (p->io_context)
+				e->ops.trim(p->io_context);
 			task_unlock(p);
 		} while_each_thread(g, p);
 		read_unlock(&tasklist_lock);