Revert "net: netprio_cgroup: make net_prio_subsys static"

This reverts commit 865d9f9f.

This commit breaks the build with CONFIG_NETPRIO_CGROUP=y so
revert it. It does build as a module though. The SUBSYS macro
in the cgroup core code automatically defines a subsys structure
as extern. Long term we should fix the macro. And I need to
fully build test things.

Tested with CONFIG_NETPRIO_CGROUP={y|m|n} with and without
CONFIG_CGROUPS defined.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
CC: Neil Horman <nhorman@tuxdriver.com>
Reported-By: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>