net: fix build errors if ipv6 is disabled

CONFIG_IPV6=n is still a valid choice ;)

It appears we can remove dead code.

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/ip6_checksum.h
Simple merge

diff --cc net/ipv4/ping.c
Simple merge

diff --cc net/ipv4/tcp_metrics.c
Simple merge

diff --cc net/sunrpc/svcsock.c
Simple merge

diff --cc security/lsm_audit.c
Simple merge