KVM: x86: directly use kvm_make_request again

A one-line wrapper around kvm_make_request is not particularly
useful. Replace kvm_mmu_flush_tlb() with kvm_make_request().

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>