KVM: x86: Use "new" memslot instead of userspace memory region

Get the number of pages directly from the new memslot instead of
computing the same from the userspace memory region when allocating
memslot metadata.  This will allow a future patch to drop @mem.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <ef44892eb615f5c28e682bbe06af96aff9ce2a9f.1638817639.git.maciej.szmigiero@oracle.com>

arch/x86/kvm/x86.c

@@ -11593,9 +11593,9 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages)
 }
 
 static int kvm_alloc_memslot_metadata(struct kvm *kvm,
-				      struct kvm_memory_slot *slot,
-				      unsigned long npages)
+				      struct kvm_memory_slot *slot)
 {
+	unsigned long npages = slot->npages;
 	int i, r;
 
 	/*
@@ -11680,8 +11680,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 				   enum kvm_mr_change change)
 {
 	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
-		return kvm_alloc_memslot_metadata(kvm, new,
-						  mem->memory_size >> PAGE_SHIFT);
+		return kvm_alloc_memslot_metadata(kvm, new);
 
 	if (change == KVM_MR_FLAGS_ONLY)
 		memcpy(&new->arch, &old->arch, sizeof(old->arch));