ipsec: pfkey should ignore events when no listeners

When pfkey has no km listeners, it still does a lot of work
before finding out there aint nobody out there.
If a tree falls in a forest and no one is around to hear it, does it make
a sound? In this case it makes a lot of noise:
With this short-circuit adding 10s of thousands of SAs using
netlink improves performance by ~10%.
Signed-off-by: NJamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/key/af_key.c

@@ -3030,6 +3030,9 @@ static int key_notify_sa_expire(struct xfrm_state *x, struct km_event *c)
 
 static int pfkey_send_notify(struct xfrm_state *x, struct km_event *c)
 {
+	if (atomic_read(&pfkey_socks_nr) == 0)
+		return 0;
+
 	switch (c->event) {
 	case XFRM_MSG_EXPIRE:
 		return key_notify_sa_expire(x, c);