ZTS: Fix test zfs_mount_006_pos
For Linux, place a file in the mount point folder so it will be
considered "busy". Fix the while loop so it doesn't rm in
directories above the testdir. Add Linux-specific code to test
overlay on|off.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #4990
Closes #8081