netns: introduce the net_hash_mix "salt" for hashes

There are many possible ways to add this "salt", thus I made this
patch to be the last in the series to change it if required.

Currently I propose to use the struct net pointer itself as this 
salt, but since this pointer is most often cache-line aligned, shift 
this right to eliminate the bits, that are most often zeroed.

After this, simply add this mix to prepared hashfn-s.

For CONFIG_NET_NS=n case this salt is 0 and no changes in hashfn
appear.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/hash.h

@@ -0,0 +1,21 @@
+#ifndef __NET_NS_HASH_H__
+#define __NET_NS_HASH_H__
+
+#include <asm/cache.h>
+
+struct net;
+
+static inline unsigned net_hash_mix(struct net *net)
+{
+#ifdef CONFIG_NET_NS
+	/*
+	 * shift this right to eliminate bits, that are
+	 * always zeroed
+	 */
+
+	return (unsigned)(((unsigned long)net) >> L1_CACHE_SHIFT);
+#else
+	return 0;
+#endif
+}
+#endif