mac802.11: only wait for nl ack if unfinished

Jon reported that the setup via netlink of a raw 802.11 mon
device fails with:

  root~/gencfg# trafgen --in beacon.cfg --rfraw --out wlan0
  Waiting for netlink ack failed!

Let us only wait for it, if really needed.

Reported-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
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>