dm ioctl: Refuse to create device named "control"

Typical userspace setups create a symlink under /dev/mapper with the
name of the device, but /dev/mapper/control is reserved for DM's control
device.  Therefore, trying to create such a device is almost certain to
be a userspace bug.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>