net-test: packetdrill: fix packet_append_header() calculations for remote mode

Fix the packet header location calculations in packet_append_header()
forgot to account for the fact that there might be layer 2 headers.

Remote mode has been broken since the addition of encapsulation
support, partly due to this issue.

Change-Id: Idfb0670da8799e11fe1b72771ed13d52d8e991fe