Fix bug in obj updater run_once().

The "not" in front of the ismount() call got accidentally dropped in a
recent change.  This patch adds it back along with a few more tests.

Note that this bug only showed up on an SAIO during probe tests because
I used actually-mounted (virtual) "disks".  So keep that in mind when
building SAIOs for development/testing.

Change-Id: Ia193f3c4b73203605954036863575c22ddab6b03
Merge "Fix bug in obj updater run_once()."