[IPV6] ADDRCONF: Mobile IPv6 Home Address support.

IFA_F_HOMEADDRESS is introduced for Mobile IPv6 Home Addresses on
Mobile Node.

The IFA_F_HOMEADDRESS flag should be set for Mobile IPv6 Home
Addresses for 2 purposes. 1) We need to check this on receipt of
Type 2 Routing Header (RFC3775 Secion 6.4), 2) We prefer Home
Address(es) in source address selection (RFC3484 Section 5 Rule 4).

Signed-off-by: Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>