zebra: dplane cancel update thread from correct pthread

This code is called from the zebra main pthread during shutdown
but the thread event is scheduled via the zebra dplane pthread.

Hence, we should be using the `thread_cancel_async()` API to
cancel the thread event on a different pthread.

This is only ever hit in the rare case that we still have work left
to do on the update queue during shutdown.

Found via zebra crash:

```
(gdb) bt
\#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
\#1  0x00007f4e4d3f7535 in __GI_abort () at abort.c:79
\#2  0x00007f4e4d3f740f in __assert_fail_base (fmt=0x7f4e4d559ee0 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x7f4e4d9071d0 "master->owner == pthread_self()",
    file=0x7f4e4d906cf8 "lib/thread.c", line=1185, function=<optimized out>) at assert.c:92
\#3  0x00007f4e4d405102 in __GI___assert_fail (assertion=assertion@entry=0x7f4e4d9071d0 "master->owner == pthread_self()", file=file@entry=0x7f4e4d906cf8 "lib/thread.c",
    line=line@entry=1185, function=function@entry=0x7f4e4d906b68 <__PRETTY_FUNCTION__.15817> "thread_cancel") at assert.c:101
\#4  0x00007f4e4d8d095a in thread_cancel (thread=0x55b40d01a640) at lib/thread.c:1185
\#5  0x000055b40c291845 in zebra_dplane_shutdown () at zebra/zebra_dplane.c:3274
\#6  0x000055b40c27ee13 in zebra_finalize (dummy=<optimized out>) at zebra/main.c:202
\#7  0x00007f4e4d8d1416 in thread_call (thread=thread@entry=0x7ffcbbc08870) at lib/thread.c:1599
\#8  0x00007f4e4d8a1ef8 in frr_run (master=0x55b40ce35510) at lib/libfrr.c:1024
\#9  0x000055b40c270916 in main (argc=8, argv=0x7ffcbbc08c78) at zebra/main.c:483
(gdb) down
\#4  0x00007f4e4d8d095a in thread_cancel (thread=0x55b40d01a640) at lib/thread.c:1185
1185		assert(master->owner == pthread_self());
(gdb)
```

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>

zebra/zebra_dplane.c

@@ -3752,7 +3752,9 @@ void zebra_dplane_shutdown(void)
 
 	zdplane_info.dg_run = false;
 
-	THREAD_OFF(zdplane_info.dg_t_update);
+	if (zdplane_info.dg_t_update)
+		thread_cancel_async(zdplane_info.dg_t_update->master,
+				    &zdplane_info.dg_t_update, NULL);
 
 	frr_pthread_stop(zdplane_info.dg_pthread, NULL);