GEOM label of a read-only partition is not properly handled

[Wojciech Puchar]

> /dev/ad10s1h or /dev/ufs/LIBRARY as special argument of mount).
>
> Was this behavior remarked by others? I have not found a PR about this, I 
> maybe should do this, right?

this is right behaviour. you may use the device many times if read-only, 
but not read-write.