KVM: Drop bogus "pfn != 0" guard from kvm_release_pfn()

Remove a check from kvm_release_pfn() to bail if the provided @pfn is
zero.  Zero is a perfectly valid pfn on most architectures, and should
not be used to indicate an error or an invalid pfn.  The bogus check was
added by commit 9172481 ("x86/kvm: Cache gfn to pfn translation"),
which also did the bad thing of zeroing the pfn and gfn to mark a cache
invalid.  Thankfully, that bad behavior was axed by commit 357a18a
("KVM: Kill kvm_map_gfn() / kvm_unmap_gfn() and gfn_to_pfn_cache").

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -2745,9 +2745,6 @@ EXPORT_SYMBOL_GPL(gfn_to_page);
 
 void kvm_release_pfn(kvm_pfn_t pfn, bool dirty)
 {
-	if (pfn == 0)
-		return;
-
 	if (dirty)
 		kvm_release_pfn_dirty(pfn);
 	else