trafgen: Remove unnecessary reinitialization of icmp packet

The entire packet is zeroed using memset() three lines above, thus there
is no need to set icmp->code and icmp->checksum to 0 explicitely again.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
tprintf: Rename parameter to avoid shadowing global declaration

The tprintf module already uses a static variable named 'buffer', thus
rename the parameter to avoid a name clash. Found by compiling with
-Wshadow.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>