sysfs: add sysfs_change_owner()

sysfs: add sysfs_change_owner()

Add a helper to change the owner of sysfs objects.
This function will be used to correctly account for kobject ownership
changes, e.g. when moving network devices between network namespaces.

This mirrors how a kobject is added through driver core which in its guts is
done via kobject_add_internal() which in summary creates the main directory via
create_dir(), populates that directory with the groups associated with the
ktype of the kobject (if any) and populates the directory with the basic
attributes associated with the ktype of the kobject (if any). These are the
basic steps that are associated with adding a kobject in sysfs.
Any additional properties are added by the specific subsystem itself (not by
driver core) after it has registered the device. So for the example of network
devices, a network device will e.g. register a queue subdirectory under the
basic sysfs directory for the network device and than further subdirectories
within that queues subdirectory.  But that is all specific to network devices
and they call the corresponding sysfs functions to do that directly when they
create those queue objects. So anything that a subsystem adds outside of what
driver core does must also be changed by it (That's already true for removal of
files it created outside of driver core.) and it's the same for ownership
changes.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>