[NET]: Add const markers to various variables.

the patch below marks various variables const in net/; the goal is to
move them to the .rodata section so that they can't false-share
cachelines with things that get written to, as well as potentially
helping gcc a bit with optimisations.  (these were found using a gcc
patch to warn about such variables)

Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 1bdf0fb..34a3322 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -751,7 +751,7 @@
 	inet6_del_protocol(&icmpv6_protocol, IPPROTO_ICMPV6);
 }
 
-static struct icmp6_err {
+static const struct icmp6_err {
 	int err;
 	int fatal;
 } tab_unreach[] = {