vnet: complete the fix for l3_hdr_offset calculation for single loop fastpass case...

vnet: complete the fix for l3_hdr_offset calculation for single loop fastpass case (VPP-1444)

20e6d36b has moved the calculation of the l3_hdr_offset into the determine_next_node()
function, with the assumption that the current_data in the buffer is at
the L3 header. This is not the case for the single loop fastpath,
where the vlib_buffer_advance() call is made after the call to
determine_next_node(), as a day1 behavior. As a result - that path
incorrectly sets the l3_hdr_offset.

Solution: move the vlib_buffer_advance() call to before determine_next_node()

Change-Id: Id5eaa084c43fb6564f8239df4a0b3dc0412b15de
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>