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 <[email protected]>
Tested-by: Antony Antony <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>

Author: choppsv1