sock: add net to prot->enter_memory_pressure callback

The tcp_enter_memory_pressure calls NET_INC_STATS, but doesn't
have where to get the net from.

I decided to add a sk argument, not the net itself, only to factor
all the required sock_net(sk) calls inside the enter_memory_pressure 
callback itself.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/decnet/af_decnet.c b/net/decnet/af_decnet.c
index 931bdf9..61b7df5 100644
--- a/net/decnet/af_decnet.c
+++ b/net/decnet/af_decnet.c
@@ -451,7 +451,7 @@
 
 static int dn_memory_pressure;
 
-static void dn_enter_memory_pressure(void)
+static void dn_enter_memory_pressure(struct sock *sk)
 {
 	if (!dn_memory_pressure) {
 		dn_memory_pressure = 1;