KVM: x86/mmu: Revert "KVM: x86/mmu: Remove is_obsolete() call"

Now that the fast invalidate mechanism has been reintroduced, restore
the performance tweaks for fast invalidation that existed prior to its
removal.

Paraphrasing the original changelog (commit 5ff05683 was itself a
partial revert):

  Don't force reloading the remote mmu when zapping an obsolete page, as
  a MMU_RELOAD request has already been issued by kvm_mmu_zap_all_fast()
  immediately after incrementing mmu_valid_gen, i.e. after marking pages
  obsolete.

This reverts commit 5ff05683.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>