generic/042: fix stale disk contents check

generic/042: fix stale disk contents check

This test doesn't call fsync or sync to force writeback of the first 60k
of the file, which means that we could end up with a file full of
zeroes or an empty file.  Since this is a regression test that looks for
stale disk contents slipping through, change the test to look for the
stale bytes.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>