ipv6: move in6_dev_finish_destroy() into core kernel

in6_dev_put() will be needed by vxlan module, so is
in6_dev_finish_destroy().

Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>