[IPV6]: ICMP6_MIB_OUTMSGS increment duplicated

icmpv6_send() calls ip6_push_pending_frames() indirectly.
Both ip6_push_pending_frames() and icmpv6_send() increment
counter ICMP6_MIB_OUTMSGS.

This patch remove the increment from icmpv6_send.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>