fuzzy: abort scrub stress testing if the scratch fs went down

fuzzy: abort scrub stress testing if the scratch fs went down

There's no point in continuing a stress test of online fsck if the
filesystem goes down.  We can't query that kind of state directly, so as
a proxy we try to stat the mountpoint and interpret any error return as
a sign that the fs is down.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>