net: add network priority cgroup infrastructure (v4)

This patch adds in the infrastructure code to create the network priority
cgroup.  The cgroup, in addition to the standard processes file creates two
control files:

1) prioidx - This is a read-only file that exports the index of this cgroup.
This is a value that is both arbitrary and unique to a cgroup in this subsystem,
and is used to index the per-device priority map

2) priomap - This is a writeable file.  On read it reports a table of 2-tuples
<name:priority> where name is the name of a network interface and priority is
indicates the priority assigned to frames egresessing on the named interface and
originating from a pid in this cgroup

This cgroup allows for skb priority to be set prior to a root qdisc getting
selected. This is benenficial for DCB enabled systems, in that it allows for any
application to use dcb configured priorities so without application modification

Signed-off-by: Neil Horman <[email protected]>
Signed-off-by: John Fastabend <[email protected]>
CC: Robert Love <[email protected]>
CC: "David S. Miller" <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/Kconfig

@@ -232,6 +232,13 @@ config XPS
 	depends on SMP && SYSFS && USE_GENERIC_SMP_HELPERS
 	default y
 
+config NETPRIO_CGROUP
+	tristate "Network priority cgroup"
+	depends on CGROUPS
+	---help---
+	  Cgroup subsystem for use in assigning processes to network priorities on
+	  a per-interface basis
+
 config HAVE_BPF_JIT
 	bool