kni: fix ioctl in container

kni: fix ioctl in container

In containers like docker, current->pid returns current process's global
PID instead of its own PID under containers's PID namespace, and
get_net_ns_by_pid() suppose to accept a virtual PID under its own
namespace, so we should use task_pid_vnr(current) to get current process's
virtual PID instead of current->pid.

Signed-off-by: Wenfeng Liu <liuwf@arraynetworks.com.cn>
Acked-by: Helin Zhang <helin.zhang@intel.com>