[dm-thin] If pool is created in read-only-metadata mode then only open the metadata device with FMODE_READ

Previously it was always opened FMODE_READ | FMODE_WRITE.

There's still an outstanding issue here:

If I create a read-only linear device for the metadata, via dmsetup
load --readonly, then you can still create a rw pool out of it.  The
dm_get_device() still succeeds.  Handing back to agk to decide how he
wants to handle this.