The __u16 queue_mapping field only makes sense in the
CONFIG_NETDEVICES_MULTIQUEUE=y case only.

Despite this field may be set explicitly to some non-zero 
value (in net/core/pktgen.c), the exact value affects 
nothing in case the config option in question is N (mainly 
it is used in netif_subqueue_stopped(), which will always
return 0 in this case).

So cleanup the code a bit and move this field under the
config option.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>