[RFC PATCH v3 2/2] L2TP:Adjust intf MTU,factor underlay L3,overlay L2

[R. Parameswaran]

[v3: Picked up review comments from James Chapman, added a
 function  to compute ip header + ip option overhead on a socket, and factored
 it  into L2TP change-set, RFC, would like early feedback on name and
 placement  of new function while I test this.

 Part 2/2: Changes in l2tp_eth.c, using the new API from part 1]