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We no longer use current_top_of_stack() on arm64, so it can be removed.

We introduced current_top_of_stack() for STACKLEAK in commit:

  0b3e3366 ("arm64: Add support for STACKLEAK gcc plugin")

... then we figured out the intended semantics were unclear, and
reworked it in commit:

  e85094c3 ("arm64: stackleak: fix current_top_of_stack()")

... then we removed the only user in commit:

  0cfa2ccd

 ("stackleak: rework stack high bound handling")

Given that it's no longer used, and it's very easy to misuse, this patch
removes current_top_of_stack(). For the moment, on_thread_stack() is
left where it is as moving it will change some header dependencies.

There should be no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221117120902.3974163-2-mark.rutland@arm.com

Signed-off-by: Will Deacon <will@kernel.org>