[IPV6] MIP6: Revert address to send ICMPv6 error.

[IPV6] MIP6: Revert address to send ICMPv6 error.

IPv6 source address is replaced in receiving packet
with home address option carried by destination options header.
To send ICMPv6 error back, original address which is received one on wire
should be used. This function checks such header is included
and reverts them.
Based on MIPL2 kernel patch.

This patch was also written by: Ville Nuorvala <vnuorval@tcs.hut.fi>

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>