[IPV6] MIP6: Add sending mobility header functions through raw socket.

Mobility header is built by user-space and sent through raw socket.
Kernel just extracts its type to flow.
Based on MIPL2 kernel patch.

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>