xfrm: Traffic Flow Confidentiality for IPv6 ESP

Add TFC padding to all packets smaller than the boundary configured
on the xfrm state. If the boundary is larger than the PMTU, limit
padding to the PMTU.

Signed-off-by: Martin Willi <martin@strongswan.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>