s390/mm: Fix VM_FAULT_HWPOISON handling in do_exception()

In the Linux kernel, the following vulnerability has been resolved:

s390/mm: Fix VM_FAULT_HWPOISON handling in do_exception()

There is no support for HWPOISON, MEMORY_FAILURE, or ARCH_HAS_COPY_MC on

s390. Therefore we do not expect to see VM_FAULT_HWPOISON in

do_exception().

However, since commit af19487f00f3 ("mm: make PTE_MARKER_SWAPIN_ERROR more

general"), it is possible to see VM_FAULT_HWPOISON in combination with

PTE_MARKER_POISONED, even on architectures that do not support HWPOISON

otherwise. In this case, we will end up on the BUG() in do_exception().

Fix this by treating VM_FAULT_HWPOISON the same as VM_FAULT_SIGBUS, similar

to x86 when MEMORY_FAILURE is not configured. Also print unexpected fault

flags, for easier debugging.

Note that VM_FAULT_HWPOISON_LARGE is not expected, because s390 cannot

support swap entries on other levels than PTE level.