netprio: use css ID instead of cgroup ID

netprio uses cgroup ID to index the priority mapping table.  This is
currently okay as cgroup IDs are allocated using idr and packed.
However, cgroup IDs will be changed to use full 64bit range and won't
be packed making this impractical.  netprio doesn't care what type of
IDs it uses as long as they can identify the controller instances and
are packed.  Let's switch to css IDs instead of cgroup IDs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Namhyung Kim <namhyung@kernel.org>