uaccess: fix integer overflow on access_ok()

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

uaccess: fix integer overflow on access_ok()

Three architectures check the end of a user access against the
address limit without taking a possible overflow into account.
Passing a negative length or another overflow in here returns
success when it should not.

Use the most common correct implementation here, which optimizes
for a constant 'size' argument, and turns the common case into a
single comparison.