vnet: ethernet-input incorrectly sets l3_hdr_offset in some cases

vnet: ethernet-input incorrectly sets l3_hdr_offset in some cases

The issue surfaced when developing the tap GSO code, with
an iteration where output path is reliant on
vnet_buffer (b0)->l3_hdr_offset being set correctly in
the input path, during performance testing.

Adding a workaround in the TX path shows that
the issue surfaces only for relatively few packets
during the test (about 100 out of 600000).

Analysis shows the issue arises if the ethernet-input
is handling two untagged packets with different sw_if_index
values - then the accelerated path punts to slow path,
before the setting of the l2.l2_len values is done,
thus resulting in them being 0, and l3_hdr_offset being
the same as l2_hdr_offset, wreaking havoc on TX path.

The solution is to move the l2_hdr_offset calculation
into a place where it is done for all the packets,
and move the l3_hdr_offset calculation into
the determine_next_node() function - as that function is
also the one setting the special-case l2.l2_len value for
tagged packets and moving the current_data for the L2 case.

Change-Id: If728c7715e011930c1887691188c98055bddde67
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>