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

[Michaël Grünewald]

Wojciech Puchar wrote:
>> /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.

Fine, thank you!