cgroups: Allow dynamically changing net_classid

The classid of a process is changed either when a process is moved to
or from a cgroup or when the net_cls.classid file is updated.
Previously net_cls only supported propogating these changes to the
cgroup's related sockets when a process was added or removed from the
cgroup. This means it was neccessary to remove and re-add all processes
to a cgroup in order to update its classid. This change introduces
support for doing this dynamically - i.e. when the value is changed in
the net_cls_classid file, this will also trigger an update to the
classid associated with all sockets controlled by the cgroup.
This mimics the behaviour of other cgroup subsystems.
net_prio circumvents this issue by storing an index into a table with
each socket (and so any updates to the table, don't require updating
the value associated with the socket). net_cls, however, passes the
socket the classid directly, and so this additional step is needed.

Signed-off-by: Nina Schiff <ninasc@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>