Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

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

Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for

__access_ok()").

This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),

because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some

computation.

The reasoning was that all user addresses are less than LONG_MAX, and all

kernel addresses are greater than LONG_MAX. Therefore access_ok() can

filter kernel addresses.

Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but

access_ok() let them pass. That was thought to be okay, because they are

not valid addresses at hardware level.

Unfortunately, one case is missed: get_user_pages_fast() happily accepts

addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses

get_user_pages_fast(). This causes the problem reported by Robert [1].

Therefore, revert this commit. TASK_SIZE_MAX is changed to the default:

TASK_SIZE.

This unfortunately reduces performance, because TASK_SIZE is more expensive

to compute compared to LONG_MAX. But correctness first, we can think about

optimization later, if required.