workaround for libpcap 4 extra bytes

Workaround for libpcap returning a length that is 4 bytes longer than
the packet on the wire. Observed on:

Linux beaglebone 3.8.13-bone50 #1 SMP Tue May 13 13:24:52 UTC 2014
armv7l GNU/Linux
ldd fwknopd
libfko.so.2 => /usr/local/lib/libfko.so.2 (0xb6f62000)
libpcap.so.0.8 => /usr/lib/arm-linux-gnueabihf/libpcap.so.0.8
(0xb6f20000)
libc.so.6 => /lib/arm-linux-gnueabihf/libc.so.6 (0xb6e3b000)
/lib/ld-linux-armhf.so.3 (0xb6f94000)
libgcc_s.so.1 => /lib/arm-linux-gnueabihf/libgcc_s.so.1 (0xb6e17000)

Calculate the new pkt_end from the length in the ip header.