datapath lisp: Use ovs offload compat functionality.

OVS already has compat functions to handle GSO packets.
Following patch get rid of GSO packet handling in lisp
and use ovs iptunnel_xmit() function for same.

CC: Lori Jakab <lojakab@cisco.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>