[IPV6] XFRM: Fix connected socket to use transformation.

When XFRM policy and state are ready after TCP connection is started,
the traffic should be transformed immediately, however it does not
on IPv6 TCP.

It depends on a dst cache replacement policy with connected socket.
It seems that the replacement is always done for IPv4, however, on
IPv6 case it is done only when routing cookie is changed.

This patch fix that non-transformation dst can be changed to
transformation one.
This behavior is required by MIPv6 and improves IPv6 IPsec.

Fixes by Masahide NAKAMURA.

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