[IPV6]: Find option offset by type.

This is a helper to search option offset from extension header which
can carry TLV option like destination options header.

Mobile IPv6 home address option will use it.

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>