btrfs: fixes for kobject changes in mainline

Here's a patch against the unstable tree that gets the code to build
against Linus's current tree (2.6.24-git12).  This is needed as the
kobject/kset api has changed there.

I tried to make the smallest changes needed, and it builds and loads
successfully, but I don't have a btrfs volume anywhere (yet) to try to
see if things still work properly :)

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>