ipv6: consolidate ipv6 sock_stat code at the beginning of net/ipv6/proc.c

Simple, comsolidate sockstat6 staff in one place, at the beginning of
the file. Right now sockstat6_seq_open/sockstat6_seq_fops looks like an
intrusion in the middle of snmp6 code.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>