powerpc: make stack walking KASAN-safe

powerpc: make stack walking KASAN-safe

Make our stack-walking code KASAN-safe by using __no_sanitize_address.
Generic code, arm64, s390 and x86 all make accesses unchecked for similar
sorts of reasons: when unwinding a stack, we might touch memory that KASAN
has marked as being out-of-bounds. In ppc64 KASAN development, I hit this
sometimes when checking for an exception frame - because we're checking
an arbitrary offset into the stack frame.

See commit f5694c8ca4c9 ("s390/kasan: avoid false positives during stack
unwind"), commit bcfeff258756 ("arm64: disable kasan when accessing
frame->fp in unwind_frame"), commit 1c5a02762b5b ("x86/dumpstack:
Prevent KASAN false positive warnings") and commit 4042f25c0437
("tracing, kasan: Silence Kasan warning in check_stack of stack_tracer").

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210614120907.1952321-1-dja@axtens.net