xfrm: Refcount destination entry on xfrm_lookup

We return a destination entry without refcount if a socket
policy is found in xfrm_lookup. This triggers a warning on
a negative refcount when freeeing this dst entry. So take
a refcount in this case to fix it.

This refcount was forgotten when xfrm changed to cache bundles
instead of policies for outgoing flows.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xfrm/xfrm_policy.c

@@ -1804,6 +1804,8 @@ restart:
 				goto no_transform;
 			}
 
+			dst_hold(&xdst->u.dst);
+
 			spin_lock_bh(&xfrm_policy_sk_bundle_lock);
 			xdst->u.dst.next = xfrm_policy_sk_bundles;
 			xfrm_policy_sk_bundles = &xdst->u.dst;