KVM: use vma_pages() helper

Use vma_pages function on vma object instead of explicit computation.

Fix the following coccicheck warning:
./virt/kvm/kvm_main.c:3526:29-35: WARNING: Consider using vma_pages
helper on vma
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Message-Id: <1632900526-119643-1-git-send-email-yang.lee@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -3523,7 +3523,7 @@ static const struct vm_operations_struct kvm_vcpu_vm_ops = {
 static int kvm_vcpu_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct kvm_vcpu *vcpu = file->private_data;
-	unsigned long pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
+	unsigned long pages = vma_pages(vma);
 
 	if ((kvm_page_in_dirty_ring(vcpu->kvm, vma->vm_pgoff) ||
 	     kvm_page_in_dirty_ring(vcpu->kvm, vma->vm_pgoff + pages - 1)) &&