KVM: selftests: Verify that reads to inaccessible guest_memfd VMAs SIGBUS

Expand the guest_memfd negative testcases for overflow and MAP_PRIVATE to
verify that reads to inaccessible memory also get a SIGBUS.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Lisa Wang <wyihan@google.com>
Tested-by: Lisa Wang <wyihan@google.com>
Link: https://lore.kernel.org/r/20251003232606.4070510-14-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>