Add ide_tape_set_media_lock() helper and convert idetape_mtioctop(),
idetape_chrdev_open() and idetape_chrdev_release() to use it.

There should be no functional changes caused by this patch (it is
OK to modify ->door_locked if idetape_create_prevent_cmd() fails).
Acked-by: NBorislav Petkov <petkovbb@gmail.com>
Signed-off-by: NBartlomiej Zolnierkiewicz <bzolnier@gmail.com>