[NETFILTER]: Move ip6_masked_addrcmp to include/net/ipv6.h

Replace netfilter's ip6_masked_addrcmp by a more efficient version
in include/net/ipv6.h to make it usable without module dependencies.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>