xfrm: iptfs: share page fragments of inner packets

When possible rather than appending secondary (aggregated) inner packets
to the fragment list, share their page fragments with the outer IPTFS
packet. This allows for more efficient packet transmission.

Signed-off-by: Christian Hopps <chopps@labn.net>
Tested-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>