fstests: allow overlay SCRATCH_DEV to be a base fs mount point

fstests: allow overlay SCRATCH_DEV to be a base fs mount point

When configure SCRATCH_DEV to a mount point (and not to a directory therein)
then user will get a false positive error in scratch tests:

 $SCRATCH_DEV=/mnt/base/scratch is mounted but not on $SCRATCH_MNT=/mnt/scratch
 Already mounted result:
 /dev/sda6 on /mnt/base/scratch type xfs (rw,relatime,attr2,inode64,noquota)

This is due to the wrong `grep -F $SCRATCH_DEV` which matches the mount
point instead of the device in that mount.
Fix _check_mounted_on() to grep the pattern "$dev on " and "$dev on $mnt"
instead of just grepping for "$dev" and "$mnt" without the " on " anchor.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>