[IPV6]: File the fingerprints off ah6->spi/esp6->spi

In theory these are opaque 32bit values.  However, we end up
allocating them sequentially in host-endian and stick unchanged
on the wire.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>