btrfs: error on missing block group when unaccounting log tree extent buffers

Currently we only log an error message if we can't find the block group
for a log tree extent buffer when unaccounting it (while freeing a log
tree). A missing block group means something is seriously wrong and we
end up leaking space from the metadata space info. So return -ENOENT in
case we don't find the block group.

CC: stable@vger.kernel.org # 6.12+
Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>