stackdepot used to call memcmp(), which compiler tools normally
instrument, therefore every lookup used to unnecessarily call instrumented
code.  This is somewhat ok in the case of KASAN, but under KMSAN a lot of
time was spent in the instrumentation.

Link: http://lkml.kernel.org/r/20171117172149.69562-1-glider@google.comSigned-off-by: NAlexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>