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>

Author: sean-jc

tools/testing/selftests/kvm/guest_memfd_test.c

@@ -84,6 +84,7 @@ static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_size)
 	mem = kvm_mmap(map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
 
 	TEST_EXPECT_SIGBUS(memset(mem, val, map_size));
+	TEST_EXPECT_SIGBUS((void)READ_ONCE(mem[accessible_size]));
 
 	for (i = 0; i < accessible_size; i++)
 		TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);