btrfs: Remove fs_info argument from convert_free_space_to_bitmaps

This function already takes a transaction handle which contains a
reference to fs_info. So use that and remove the extra argument.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>