KVM: Use new smp_call_function_mask() in kvm_flush_remote_tlbs()

In kvm_flush_remote_tlbs(), replace a loop using smp_call_function_single()
by a single call to smp_call_function_mask() (which is new for x86_64).

Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>