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: 78c98f90 ("s390/unwind: introduce stack unwind API")
Reported-by: Julian Wiedmann <jwi@linux.ibm.com>
Tested-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>