flowprobe: fix tx flows generated for rewritten traffic

flowprobe: fix tx flows generated for rewritten traffic

Currently, when IPFIX records generation is enabled for an interface in
the TX direction, some rewritten traffic is being sent from that
interface, and the Ethernet header's location has changed due to
rewriting, generated TX flows will contain fields with wrong and zero
values. For example, that can be observed when traffic is rewritten from
a subinterface to a hardware interface (i.e. when tags are removed). A
TX flow generated in this case will have wrong L2 fields because of an
incorrectly located Ethernet header. And zero L3/L4 fields because the
Ethernet type will match neither IP4 nor IP6.

The same code is executed to generate flows for both input and output
features. And the same mechanism is applied to identify the Ethernet
header in the buffer's data. However, such general code usually works
with the buffer's data conditionally based on the direction. For most
input features, the buffer's current_data will likely point to the IP
header. For most output features, the buffer's current_data will likely
point to the Ethernet header.

With this fix:
 - Keep relying on ethernet_buffer_get_header() to locate the Ethernet
   header for input features. And start using vlib_buffer_get_current()
   to locate the Ethernet header for output features. The function will
   account for the Ethernet header's position change in the buffer's
   data if there is rewriting.

 - After fixing Ethernet header determination in the buffer's data,
   L3/L4 fields will contain non-zero but still incorrect data. That is
   because IP header determination needs to be fixed too. It currently
   relies on the fact that the Ethernet header is always located at the
   beginning of the buffer's data and that l2_hdr_sz can be used as an
   IP header offset. However, this may not be the case after rewriting.
   So start calculating the actual offset of the IP header in the
   buffer's data.

 - Add a unit test to cover the case.

Type: fix
Change-Id: Icf3f9e6518912d06dff0d5aa48e103b3dc94edb7
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
(cherry picked from commit 64d6463d2eac0c0fe434f3a7aa56fe4d85c046d9)