xfrm: Fix refcount imbalance in xfrm_lookup

xfrm_lookup must return a dst_entry with a refcount for the caller.
Git commit 1a1ccc96abb ("xfrm: Remove caching of xfrm_policy_sk_bundles")
removed this refcount for the socket policy case accidentally.
This patch restores it and sets DST_NOCACHE flag to make sure
that the dst_entry is freed when the refcount becomes null.

Fixes: 1a1ccc96abb ("xfrm: Remove caching of xfrm_policy_sk_bundles")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -2097,6 +2097,8 @@ struct dst_entry *xfrm_lookup(struct net *net, struct dst_entry *dst_orig,
 				goto no_transform;
 			}
 
+			dst_hold(&xdst->u.dst);
+			xdst->u.dst.flags |= DST_NOCACHE;
 			route = xdst->route;
 		}
 	}