[NETNS][FRAGS]: Make the mem counter per-namespace.

This is also simple, but introduces more changes, since
then mem counter is altered in more places.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ip.h b/include/net/ip.h
index fbe2830..9f50d4f 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -331,7 +331,7 @@
 };
 
 int ip_defrag(struct sk_buff *skb, u32 user);
-int ip_frag_mem(void);
+int ip_frag_mem(struct net *net);
 int ip_frag_nqueues(struct net *net);
 
 /*