KVM: x86: Avoid gratuitous global clock reload in kvm_arch_vcpu_load()

KVM: x86: Avoid gratuitous global clock reload in kvm_arch_vcpu_load()

Commit d98d07ca7e034 ("KVM: x86: update pvclock area conditionally, on
cpu migration") turned an unconditional KVM_REQ_CLOCK_UPDATE into a
conditional one, if either the master clock isn't enabled *or* the vCPU
was not previously scheduled (vcpu->cpu == -1). The commit message doesn't
explain the latter condition, which is specifically for the master clock
case.

Commit 0061d53daf26f ("KVM: x86: limit difference between kvmclock
updates") later turned that into a KVM_REQ_GLOBAL_CLOCK_UPDATE to avoid
skew between vCPUs.

In master clock mode there is no need for any of that, regardless of
whether/where this vCPU was previously scheduled.

Do it only if (!kvm->arch.use_master_clock).

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>