ipv6: Make ipv6_is_mld() inline and use it from ip6_mc_input().

Move generalized version of ipv6_is_mld() to header,
and use it from ip6_mc_input().

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