kasan, arm64: unpoison stack only with CONFIG_KASAN_STACK

kasan, arm64: unpoison stack only with CONFIG_KASAN_STACK

There's a config option CONFIG_KASAN_STACK that has to be enabled for
KASAN to use stack instrumentation and perform validity checks for
stack variables.

There's no need to unpoison stack when CONFIG_KASAN_STACK is not enabled.
Only call kasan_unpoison_task_stack[_below]() when CONFIG_KASAN_STACK is
enabled.

Note, that CONFIG_KASAN_STACK is an option that is currently always
defined when CONFIG_KASAN is enabled, and therefore has to be tested
with #if instead of #ifdef.

Link: https://lkml.kernel.org/r/d09dd3f8abb388da397fd11598c5edeaa83fe559.1606162397.git.andreyknvl@google.com
Link: https://linux-review.googlesource.com/id/If8a891e9fe01ea543e00b576852685afec0887e3
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Branislav Rankov <Branislav.Rankov@arm.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>