dm: fix opening log and cow devices for read only tables

If a table is read-only, also open any log and cow devices it uses read-only.

Previously, even read-only devices were opened read-write internally.
After patch 75f1dc0d
  block: check bdev_read_only() from blkdev_get()
was applied, loading such tables began to fail.  The patch
was reverted by e51900f7
  block: revert block_dev read-only check
but this patch fixes this part of the code to work with the original patch.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>