af_packet: set next0 for AF_PACKET_IF_MODE_ETHERNET mode

af_packet: set next0 for AF_PACKET_IF_MODE_ETHERNET mode

Normally af_packet sets next0 = next_index on each cycle. It works for the most cases.
But if vlib_validate_buffer_enqueue_x1() changes the next_index (from NEXT_ETHERNET to NEXT_DROP for example)
then the following next0 will have the wrong value, and the correct packet will be dropped.

AF_PACKET_IF_MODE_IP handles this case, but AF_PACKET_IF_MODE_ETHERNET doesn't.

Type: fix

Signed-off-by: Artem Glazychev <artem.glazychev@xored.com>
Change-Id: Ic742043e8b10a2abe56b314bb584277151a9c5eb