If the unwinder somehow ends up with a stack pointer that lies outside
the stack, then an attempt to dereference can lead to a fault, which
causes the kernel to panic again and unwind the stack, which leads to a
fault...

Add kstack_contains() checks at points where we dereference the stack
pointer. This avoids the aforementioned infinite loop in one case I
have where some openssl assembly code apparently confuses the unwinder.

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