cgroup: netprio: remove unnecessary task_netprioidx

Since the tasks have been migrated to the cgroup,
there is no need to call task_netprioidx to get
task's cgroup id.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>