[IPV6] RAW: Add checksum default defines for MH.

Add checksum default defines for mobility header(MH) which
goes through raw socket. As the result kernel's behavior is
to handle MH checksum as default.

This patch also removes verifying inbound MH checksum at
mip6_mh_filter() since it did not consider user specified
checksum offset and was redundant check with raw socket code.

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