mdcheck: when mdcheck_start is enabled, enable mdcheck_continue too.

mdcheck: when mdcheck_start is enabled, enable mdcheck_continue too.

mdcheck_continue continues a regular array scan that was started by
mdcheck_start.
mdcheck_start will ensure that mdcheck_continue is active.
Howver if you reboot after a check has started, but before it finishes,
then mdcheck_continue won't cause it to continue, because nothing
starts it on boot.

So add an install option for mdcheck_contine, and make sure it
gets enabled when mdcheck_start is enabled.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>