KVM: selftests: fix unintentional noop test in guest_memfd_test.c

The loop in test_create_guest_memfd_invalid() that is supposed to test
that nothing is accepted as a valid flag to KVM_CREATE_GUEST_MEMFD was
initializing `flag` as 0 instead of BIT(0). This caused the loop to
immediately exit instead of iterating over BIT(0), BIT(1), ... .

Fixes: 8a89efd ("KVM: selftests: Add basic selftest for guest_memfd()")
Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
Reviewed-by: James Gowans <jgowans@amazon.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20241024095956.3668818-1-roypat@amazon.co.uk
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: roypat

tools/testing/selftests/kvm/guest_memfd_test.c

@@ -134,7 +134,7 @@ static void test_create_guest_memfd_invalid(struct kvm_vm *vm)
 			    size);
 	}
 
-	for (flag = 0; flag; flag <<= 1) {
+	for (flag = BIT(0); flag; flag <<= 1) {
 		fd = __vm_create_guest_memfd(vm, page_size, flag);
 		TEST_ASSERT(fd == -1 && errno == EINVAL,
 			    "guest_memfd() with flag '0x%lx' should fail with EINVAL",