riscv: Define TASK_SIZE_MAX for __access_ok()

TASK_SIZE_MAX should be set to a constant value, at least the largest
valid userspace address under any runtime configuration. This optimizes
the check in __access_ok(), which no longer needs to compute the runtime
value of TASK_SIZE. The check does not need to be exact, as long as it
accepts all valid userspace addresses and rejects all valid kernel
addresses; well-behaved programs will never fail the access_ok() check.

For RISC-V, which requires all virtual addresses to be sign extended,
the optimal choice is LONG_MAX because it simplifies the limit
comparison to a sign bit test.

This removes about half of the references to pgtable_l[45]_enabled.

Signed-off-by: Samuel Holland <[email protected]>
Reviewed-by: Arnd Bergmann <[email protected]>
Reviewed-by: Alexandre Ghiti <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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