s390/kasan: avoid false positives during stack unwind

s390/kasan: avoid false positives during stack unwind

Avoid kasan false positive when current task is interrupted in-between
stack frame allocation and backchain write instructions leaving new stack
frame backchain invalid. In particular if backchain is 0 the unwinder
tries to read pt_regs from the stack and might hit kasan poisoned bytes,
leading to kasan "stack-out-of-bounds" report.

Disable kasan instrumentation of unwinder stack reads, since this
limitation couldn't be handled otherwise with current backchain unwinder
implementation.

Fixes: 78c98f907413 ("s390/unwind: introduce stack unwind API")
Reported-by: Julian Wiedmann <[email protected]>
Tested-by: Benjamin Block <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>