netprio_cgroup: Fix build break

I broke the build with the addition of netprio_cgroups if CONFIG_CGROUPS=n.
This patch corrects it by moving the offending struct into an ifdef
CONFIG_CGROUPS block.  Also clean up a few needless defines and inline functions
that don't get called if CONFIG_CGROUPS isn't defined while Im at it.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>