btrfs: convert the buffer_radix to an xarray

In order to fully utilize xarray tagging to improve writeback we need to
convert the buffer_radix to a proper xarray.  This conversion is
relatively straightforward as the radix code uses the xarray underneath.
Using xarray directly allows for quite a lot less code.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>