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: 8a89efd43423 ("KVM: selftests: Add basic selftest for guest_memfd()")
Signed-off-by: Patrick Roy <[e-mail]>
Reviewed-by: James Gowans <[e-mail]>
Reviewed-by: Muhammad Usama Anjum <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/guest_memfd_test.c

···
 			    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",