KVM: Do not zero initialize 'pfn' in hva_to_pfn()

Drop the unnecessary initialization of the local 'pfn' variable in
hva_to_pfn().  First and foremost, '0' is not an invalid pfn, it's a
perfectly valid pfn on most architectures.  I.e. if hva_to_pfn() were to
return an "uninitializd" pfn, it would actually be interpeted as a legal
pfn by most callers.

Second, hva_to_pfn() can't return an uninitialized pfn as hva_to_pfn()
explicitly sets pfn to an error value (or returns an error value directly)
if a helper returns failure, and all helpers set the pfn on success.

The zeroing of 'pfn' was introduced by commit 2fc84311 ("KVM:
reorganize hva_to_pfn"), probably to avoid "uninitialized variable"
warnings on statements that return pfn.  However, no compiler seems
to produce them, making the initialization unnecessary.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -2609,7 +2609,7 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
 		     bool write_fault, bool *writable)
 {
 	struct vm_area_struct *vma;
-	kvm_pfn_t pfn = 0;
+	kvm_pfn_t pfn;
 	int npages, r;
 
 	/* we can do it either atomically or asynchronously, not both */