xfrm: iptfs: add fragmenting of larger than MTU user packets

Add support for tunneling user (inner) packets that are larger than the
tunnel's path MTU (outer) using IP-TFS fragmentation.

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>