[NET/IPV4/IPV6]: Change some sysctl variables to __read_mostly

Change net/core, ipv4 and ipv6 sysctl variables to __read_mostly.

Couldn't actually measure any performance increase while testing (.3%
I consider noise), but seems like the right thing to do.

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 1030551..e3a8e27 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -151,7 +151,7 @@
 	return 0;
 }
 
-static int sysctl_icmpv6_time = 1*HZ; 
+static int sysctl_icmpv6_time __read_mostly = 1*HZ;
 
 /* 
  * Check the ICMP output rate limit