[TCP]: Improve ipv4 established hash function.

If all of the entropy is in the local and foreign addresses,
but xor'ing together would cancel out that entropy, the
current hash performs poorly.

Suggested by Cosmin Ratiu:

Basically, the situation is as follows: There is a client
machine and a server machine. Both create 15000 virtual
interfaces, open up a socket for each pair of interfaces and
do SIP traffic. By profiling I noticed that there is a lot of
time spent walking the established hash chains with this
particular setup.

The addresses were distributed like this: client interfaces
were 198.18.0.1/16 with increments of 1 and server interfaces
were 198.18.128.1/16 with increments of 1. As I said, there
were 15000 interfaces. Source and destination ports were 5060
for each connection. So in this case, ports don't matter for
hashing purposes, and the bits from the address pairs used
cancel each other, meaning there are no differences in the
whole lot of pairs, so they all end up in the same hash chain.

Signed-off-by: David S. Miller <[e-mail]>

include/net/inet_sock.h

···
 static inline unsigned int inet_ehashfn(const __be32 laddr, const __u16 lport,
 					const __be32 faddr, const __be16 fport)
 {
-	return jhash_2words((__force __u32) laddr ^ (__force __u32) faddr,
+	return jhash_3words((__force __u32) laddr,
+			    (__force __u32) faddr,
 			    ((__u32) lport) << 16 | (__force __u32)fport,
 			    inet_ehash_secret);
 }