x86/mm, kasan: Silence KASAN warnings in get_wchan()

x86/mm, kasan: Silence KASAN warnings in get_wchan()

get_wchan() is racy by design, it may access volatile stack
of running task, thus it may access redzone in a stack frame
and cause KASAN to warn about this.

Use READ_ONCE_NOCHECK() to silence these warnings.

Reported-by: Sasha Levin <[email protected]>
Signed-off-by: Andrey Ryabinin <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Kostya Serebryany <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Wolfram Gloger <[email protected]>
Cc: kasan-dev <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>