stn-plugin: do not assume all punted packets start with L3 header.

stn-plugin: do not assume all punted packets start with L3 header.

Some punt scenarios (notably, involving DNS traffic)
do not have the current_data set to the L3 header -
as a result, chaos ensues. To tackle this, approach
the parsing from the other side, and look at
the hopefully remaining ethernet header
to see whether the packet is IPv4 or IPv6.

Verified the STN'ed TCP traffic continues to work,
and that the STN'ed DNS traffic starts to work as well.

Change-Id: I0aa2ad1df2fb23dd4e54a564714103b19114d636
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>