common: exit only if tests find corruption

common: exit only if tests find corruption

If _check_xxx_filesystem called by tests when corruption found
they exit, but we don't want the calls from the test harness itself to
exit when corruption is found.

_check_xfs_filesystem already behaves correctly, make
_check_generic_filesystem and _check_btrfs_filesystem follow the same
behavior.

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>