btrfs: manage metadata_ratio mount option as %u

As metadata_ratio mount option is unsinged so manage it as %u for token
verifications, instead of %d.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>