KVM: x86: Defer MMU sync on PCID invalidation

Defer the MMU sync on PCID invalidation so that multiple sync requests in
a single VM-Exit are batched.  This is a very minor optimization as
checking for unsync'd children is quite cheap.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210609234235.1244004-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>