KVM: Don't kfree(NULL) on kzalloc() failure in kvm_assign_ioeventfd_idx()

On kzalloc() failure, taking the `goto fail` path leads to kfree(NULL).
Such no-op has no use. Move it out.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20230327175457.735903-1-mhal@rbox.co


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