CDC-NCM: avoid overflow in sanity checking

In the Linux kernel, the following vulnerability has been resolved:

CDC-NCM: avoid overflow in sanity checking

A broken device may give an extreme offset like 0xFFF0

and a reasonable length for a fragment. In the sanity

check as formulated now, this will create an integer

overflow, defeating the sanity check. Both offset

and offset + len need to be checked in such a manner

that no overflow can occur.

And those quantities should be unsigned.