generic: test writeback error handling on dmerror devices

Ensure that we get an error back on all fds when a block device is
open by multiple writers and writeback fails.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
3 files changed