This function is not really specific to the genhd layer and there are various
re-implementations or open-coded variants of it all throughout the kernel. To
avoid further duplications move the function to a more generic place.

While moving also convert it from a macro to a inline function.

Potential users of this function can be detected and converted using the
following coccinelle patch:

// <smpl>
@@
expression k;
@@
-container_of(k, struct device, kobj)
+kobj_to_dev(kobj)
// </smpl>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>