[IPV6] MROUTE: Add stats in multicast routing module method ip6_mr_forward().

This patches adds a call to increment IPSTATS_MIB_OUTFORWDATAGRAMS
when forwarding the packet in ip6_mr_forward() in the IPv6 multicast
routing module (net/ipv6/ip6mr.c).

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>