[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/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 1b9b674..81b2795 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -83,7 +83,7 @@
 #include <linux/netlink.h>
 #include <linux/tcp.h>
 
-int sysctl_ip_default_ttl = IPDEFTTL;
+int sysctl_ip_default_ttl __read_mostly = IPDEFTTL;
 
 /* Generate a checksum for an outgoing IP datagram. */
 __inline__ void ip_send_check(struct iphdr *iph)