randomize_kstack: Remove non-functional per-arch entropy filtering

An unintended consequence of commit 9c573cd31343 ("randomize_kstack:
Improve entropy diffusion") was that the per-architecture entropy size
filtering reduced how many bits were being added to the mix, rather than
how many bits were being used during the offsetting. All architectures
fell back to the existing default of 0x3FF (10 bits), which will consume
at most 1KiB of stack space. It seems that this is working just fine,
so let's avoid the confusion and update everything to use the default.

The prior intent of the per-architecture limits were:

  arm64: capped at 0x1FF (9 bits), 5 bits effective
  powerpc: uncapped (10 bits), 6 or 7 bits effective
  riscv: uncapped (10 bits), 6 bits effective
  x86: capped at 0xFF (8 bits), 5 (x86_64) or 6 (ia32) bits effective
  s390: capped at 0xFF (8 bits), undocumented effective entropy

Current discussion has led to just dropping the original per-architecture
filters. The additional entropy appears to be safe for arm64, x86,
and s390. Quoting Arnd, "There is no point pretending that 15.75KB is
somehow safe to use while 15.00KB is not."

Co-developed-by: Yuntao Liu <[email protected]>
Signed-off-by: Yuntao Liu <[email protected]>
Fixes: 9c573cd31343 ("randomize_kstack: Improve entropy diffusion")
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Arnd Bergmann <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Acked-by: Heiko Carstens <[email protected]> # s390
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

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