generic/648: dmerror must be unmounted

generic/648: dmerror must be unmounted

Sometimes g/648 fail to unmount dmerror with this error:

   umount: /mnt/xfstests/scratch: target is busy.

Even worse, it will cause all later test cases fail as:

  mount: bad usage
  Try 'mount --help' for more information.
  check: failed to mount $SCRATCH_DEV using specified options
  Interrupted!

So we shouldn't _fail directly if dmerror_unmount fails, use a while
loop to try to unmount it enough times.

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