drm/i915/gvt: Put the page reference obtained by KVM's gfn_to_pfn()

Put the struct page reference acquired by gfn_to_pfn(), KVM's API is that
the caller is ultimately responsible for dropping any reference.

Note, kvm_release_pfn_clean() ensures the pfn is actually a refcounted
struct page before trying to put any references.

Fixes: b901b252 ("drm/i915/gvt: Add 2M huge gtt support")
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://lore.kernel.org/r/20230729013535.1070024-6-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

drivers/gpu/drm/i915/gvt/gtt.c

@@ -1158,6 +1158,7 @@ static int is_2MB_gtt_possible(struct intel_vgpu *vgpu,
 {
 	const struct intel_gvt_gtt_pte_ops *ops = vgpu->gvt->gtt.pte_ops;
 	kvm_pfn_t pfn;
+	int ret;
 
 	if (!HAS_PAGE_SIZES(vgpu->gvt->gt->i915, I915_GTT_PAGE_SIZE_2M))
 		return 0;
@@ -1171,7 +1172,9 @@ static int is_2MB_gtt_possible(struct intel_vgpu *vgpu,
 	if (!pfn_valid(pfn))
 		return -EINVAL;
 
-	return PageTransHuge(pfn_to_page(pfn));
+	ret = PageTransHuge(pfn_to_page(pfn));
+	kvm_release_pfn_clean(pfn);
+	return ret;
 }
 
 static int split_2MB_gtt_entry(struct intel_vgpu *vgpu,