dm raid: add support for resync w/o metadata devices

Target does not honour the "sync" argument when activated w/o metadata
devices, e.g. with table line:
"0 $(blockdev --getsz $data1) raid raid1 2 0 sync 2 - $data1 - $data2".

Fix this to support temporary, transient raid devices useful
for data duplication.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>