Add Mobility header definition for Mobile IPv6.
Based on MIPL2 kernel patch.

This patch was also written by: Antti Tuominen <anttit@tcs.hut.fi>
Signed-off-by: NMasahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>