The nandsim(4) simulator driver doesn't have any protection against
races at least in its ioctl handler, and at the same time it creates
device entry with 0666 permissions.

To plug possible issues in it:

• Mark it as needing Giant.
• Switch device mode to 0600.

Submitted by: C Turt
Reviewed by: imp
MFC after: 1 week
Security: Possible double free in ioctl handler