xfrm6: fix inet6_dev refcount underflow problem

There are race conditions that may lead to inet6_dev refcount underflow
in xfrm6_dst_destroy() and rt6_uncached_list_flush_dev().

One of the refcount underflow bugs is shown below:
	(cpu 1)                	|	(cpu 2)
xfrm6_dst_destroy()             |
  ...                           |
  in6_dev_put()                 |
				|  rt6_uncached_list_flush_dev()
  ...				|    ...
				|    in6_dev_put()
  rt6_uncached_list_del()       |    ...
  ...                           |

xfrm6_dst_destroy() calls rt6_uncached_list_del() after in6_dev_put(),
so rt6_uncached_list_flush_dev() has a chance to call in6_dev_put()
again for the same inet6_dev.

Fix it by moving in6_dev_put() after rt6_uncached_list_del() in
xfrm6_dst_destroy().

Fixes: 510c321 ("xfrm: reuse uncached_list to track xdsts")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/xfrm6_policy.c

@@ -117,10 +117,10 @@ static void xfrm6_dst_destroy(struct dst_entry *dst)
 {
 	struct xfrm_dst *xdst = (struct xfrm_dst *)dst;
 
-	if (likely(xdst->u.rt6.rt6i_idev))
-		in6_dev_put(xdst->u.rt6.rt6i_idev);
 	dst_destroy_metrics_generic(dst);
 	rt6_uncached_list_del(&xdst->u.rt6);
+	if (likely(xdst->u.rt6.rt6i_idev))
+		in6_dev_put(xdst->u.rt6.rt6i_idev);
 	xfrm_dst_destroy(xdst);
 }