msm: ipa: L2TP fixes

1) Make changes to get mtu of l2tp interface along with
other configuration parameters to install the MTU rule configured
by user.

2) In case of L2TP over UDP, there can be packets with frag extension
header which do not have offset or MF bit set. These packets will not
match with IS_FRAG equation which relies on frag offset and MF bit set.
Make changes to give provision to install next header matching the
protocol byte offset in IPv6 header.

Change-Id: I6ae99edecd0f2cdc692fdf1de7b57d346679d2f1
Signed-off-by: Chaitanya Pratapa <cpratapa@codeaurora.org>