um: Prevent KASAN splats in dump_stack()

Use READ_ONCE_NOCHECK() when reading the stack to prevent KASAN splats
when dump_stack() is used.

Fixes: 5b301409

 ("UML: add support for KASAN under x86_64")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>