dm-writecache patches

Here I'm sending the current dm-writecache patch.
The per-architecture switch was removed.

Some reported bugs are fixed.

Mikulas

I've staged dm-writecache in linux-dm.git's dm-4.18 branch. Please rebase ontop of that.

In particular, as I said in an earlier email, I'd really appreciate an incremental patch that removes the per-architecture flushing.

Thanks,
Mike