[dm-devel] enforce read-only state at the block layer

[Christoph Hellwig]

> Not sure of a crafty hack to workaround. Hopefully 5 year old lvm2
> remains tightly coupled to kernels of the same vintage and we get
> lucky moving forward.
> 
> So I agree with Linus, worth trying this simple change again and
> seeing if there is fallout. Revert/worry about it again as needed.

I'm actually looking into implementing Linus' suggestion now and
track in the block_device if it has ever been opened for writing.