mount: flush out cycle state on DEAD→MOUNTED only, not the other way round

mount: flush out cycle state on DEAD→MOUNTED only, not the other way round

For services (and other units) we generally follow the rule that at the
beginning of each cycle, i.e. when the INACTIVE/FAILED state is left for
ACTIVATING/ACTIVE we flush out various state variables. Mount units
handled this differently so far when the unit state change was effected
outside of systemd: in that case these variables would be flushed out
when going back to INACTIVE/FAILED already.

Let's fix that, and flush out this state always during the activating
transition, not during the deactivating transition.