… in the btrfs_root struct and adjust all usages of this object to use
the XArray API, because it is notionally easier to use and understand,
as it provides array semantics, and also takes care of locking for us,
further simplifying the code.

Also use the opportunity to do some light refactoring.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NGabriel Niebler <gniebler@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>