cgroup controller API will be converted to primarily use struct
cgroup_subsys_state instead of struct cgroup.  In preparation, make
the internal functions of netprio_cgroup pass around @css instead of
@cgrp.

While at it, kill struct cgroup_netprio_state which only contained
struct cgroup_subsys_state without serving any purpose.  All functions
are converted to deal with @css directly.

This patch shouldn't cause any behavior differences.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>