libnvdimm: Fix security issue with DSM IOCTL.

libnvdimm: Fix security issue with DSM IOCTL.

BugLink: http://bugs.launchpad.net/bugs/1572722
commit 07accfa9d1a8bac8262f6d24a94a54d2d1f35149 upstream.

Code attempts to prevent certain IOCTL DSM from being called
when device is opened read only.  This security feature can
be trivially overcome by changing the size portion of the
ioctl_command which isn't used.

Check only the _IOC_NR (i.e. the command).

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>