kni: create interface in current network namespace

kni: create interface in current network namespace

With this patch, KNI interface (e.g. vEth0) is created in the
network namespace where the DPDK application is running.
Otherwise, all interfaces are created in the default namespace
in the host.

put_net() is required, since get_net_ns_by_pid() increments
the reference counter of the network namespace with get_net().

Signed-off-by: Takayuki Usui <takayuki@midokura.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>