btrfs: sysfs: add debugging exports

Add 'debug' directories to global sysfs and per-filesystem. This will
replace the debugfs directory. The sysfs location is simpler and builds
on top of the existing file hierarchy so there will hopefully be no more
questions about the sample debugfs file.

The directory is called 'debug' and only present under
CONFIG_BTRFS_DEBUG so this will not affect productions builds.

Signed-off-by: David Sterba <dsterba@suse.com>