ipv6: consistent use of IP6_INC_STATS_BH() in ip6_forward()

ip6_forward() runs from softirq context, we can use the SNMP macros
assuming this.

Use same indentation for all IP6_INC_STATS_BH() calls.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>