btrfs: use bool argument in free_root_pointers()

We don't need int argument bool shall do in free_root_pointers().  And
rename the argument as it confused two people.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>