KVM: Clean up __kvm_gfn_to_hva_cache_init() and its callers

Barret reported a (technically benign) bug where nr_pages_avail can be
accessed without being initialized if gfn_to_hva_many() fails.

  virt/kvm/kvm_main.c:2193:13: warning: 'nr_pages_avail' may be
  used uninitialized in this function [-Wmaybe-uninitialized]

Rather than simply squashing the warning by initializing nr_pages_avail,
fix the underlying issues by reworking __kvm_gfn_to_hva_cache_init() to
return immediately instead of continuing on.  Now that all callers check
the result and/or bail immediately on a bad hva, there's no need to
explicitly nullify the memslot on error.
Reported-by: Barret Rhoden <brho@google.com>
Fixes: f1b9dd5e ("kvm: Disallow wraparound in kvm_gfn_to_hva_cache_init")
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -2130,33 +2130,36 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
 	gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
 	gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
 	gfn_t nr_pages_avail;
-	int r = start_gfn <= end_gfn ? 0 : -EINVAL;
 
-	ghc->gpa = gpa;
+	/* Update ghc->generation before performing any error checks. */
 	ghc->generation = slots->generation;
-	ghc->len = len;
+
+	if (start_gfn > end_gfn) {
 		ghc->hva = KVM_HVA_ERR_BAD;
+		return -EINVAL;
+	}
 
 	/*
 	 * If the requested region crosses two memslots, we still
 	 * verify that the entire region is valid here.
 	 */
-	while (!r && start_gfn <= end_gfn) {
+	for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) {
 		ghc->memslot = __gfn_to_memslot(slots, start_gfn);
 		ghc->hva = gfn_to_hva_many(ghc->memslot, start_gfn,
 					   &nr_pages_avail);
 		if (kvm_is_error_hva(ghc->hva))
-			r = -EFAULT;
-		start_gfn += nr_pages_avail;
+			return -EFAULT;
 	}
 
 	/* Use the slow path for cross page reads and writes. */
-	if (!r && nr_pages_needed == 1)
+	if (nr_pages_needed == 1)
 		ghc->hva += offset;
 	else
 		ghc->memslot = NULL;
 
-	return r;
+	ghc->gpa = gpa;
+	ghc->len = len;
+	return 0;
 }
 
 int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,