KVM: x86: Don't call kvm_mmu_change_mmu_pages() if the count hasn't changed

There is no point in calling kvm_mmu_change_mmu_pages() for memslot
operations that don't change the total page count, so do it just for
KVM_MR_CREATE and KVM_MR_DELETE.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <9e56b7616a11f5654e4ab486b3237366b7ba9f2a.1638817640.git.maciej.szmigiero@oracle.com>