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: 8a89efd4 ("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>