[IPV6]: Consolidate common SNMP code

This patch moves the non-proc SNMP code into addrconf.c and reuses
IPv4 SNMP code where applicable.

As a result we can skip proc.o if /proc is disabled.

Note that I've made a number of functions static since they're only
used by addrconf.c for now.  If they ever get used elsewhere we can
always remove the static.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>