[XFRM] IPV6: Restrict bundle reusing

[XFRM] IPV6: Restrict bundle reusing

For outbound transformation, bundle is checked whether it is
suitable for current flow to be reused or not. In such IPv6 case
as below, transformation may apply incorrect bundle for the flow instead
of creating another bundle:

- The policy selector has destination prefix length < 128
  (Two or more addresses can be matched it)
- Its bundle holds dst entry of default route whose prefix length < 128
  (Previous traffic was used such route as next hop)
- The policy and the bundle were used a transport mode state and
  this time flow address is not matched the bundled state.

This issue is found by Mobile IPv6 usage to protect mobility signaling
by IPsec, but it is not a Mobile IPv6 specific.
This patch adds strict check to xfrm_bundle_ok() for each
state mode and address when prefix length is less than 128.

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>