[PKTGEN]: fix device name handling

Since devices can change name and other wierdness, don't hold onto
a copy of device name, instead use pointer to output device.

Fix a couple of leaks in error handling path as well.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>