repair-size: ensure child disks have sane sizes

While this patch doesn't do a full match of on-disk size to config-size
for child devices, it does a sanity check (for DRBD only) that the child
size is not less than the DRBD size. While this would be a strange
occurrence (failed growth and masterd crash or similar), it does prevent
replace-disks from working.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>