btrfs-progs: scrub, detect stale information in the status file

If scrub is not cancelled nor finished, the recorded status will prevent
scrub to start again though it's not running. There's a force option to
run it anyway, but this is just a bandaid and the true status of scrub
should be detected automatically. The force option should not be
necessary anymore.

The test introduced in 9681f82853360aac1ff2 checks only the status file,
not kernel status of scrub.

Signed-off-by: David Sterba <dsterba@suse.cz>