KVM: arm/arm64: Signal SIGBUS when stage2 discovers hwpoison memory

Once we enable ARCH_SUPPORTS_MEMORY_FAILURE on arm64, notifications for
broken memory can call memory_failure() in mm/memory-failure.c to deliver
SIGBUS to any user space process using the page, and notify all the
in-kernel users.

If the page corresponded with guest memory, KVM will unmap this page
from its stage2 page tables. The user space process that allocated
this memory may have never touched this page in which case it may not
be mapped meaning SIGBUS won't be delivered.

This works well until a guest accesses that page, and KVM discovers
pfn == KVM_PFN_ERR_HWPOISON when it comes to process the stage2 fault.

Do as x86 does, and deliver the SIGBUS when we discover
KVM_PFN_ERR_HWPOISON. Use the stage2 mapping size as the si_addr_lsb
as this matches the user space mapping size.

Signed-off-by: James Morse <james.morse@arm.com>