trafgen: fix packet socket initialization with multiple CPUs

trafgen: fix packet socket initialization with multiple CPUs

The commit 78c13b71e196 ("trafgen: Allow to generate packets
to output pcap file") introduced a regression when output is
a network device and multiple CPU are in use: the packet
socket is created before fork() and thus the socket is shared
among all the processes: all of them except the first will
fail while setting the tx_ring.

Fix it splitting the io open() helper in a create() op,
called before forking, and the open() op called by each process.

Fixes: 78c13b71e196 ("trafgen: Allow to generate packets to output pcap file")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>