KVM: downgrade two BUG_ONs to WARN_ON_ONCE

This is not an unrecoverable situation.  Users of kvm_read_guest_offset_cached
and kvm_write_guest_offset_cached must expect the read/write to fail, and
therefore it is possible to just return early with an error value.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -2931,7 +2931,8 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	int r;
 	gpa_t gpa = ghc->gpa + offset;
 
-	BUG_ON(len + offset > ghc->len);
+	if (WARN_ON_ONCE(len + offset > ghc->len))
+		return -EINVAL;
 
 	if (slots->generation != ghc->generation) {
 		if (__kvm_gfn_to_hva_cache_init(slots, ghc, ghc->gpa, ghc->len))
@@ -2968,7 +2969,8 @@ int kvm_read_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
 	int r;
 	gpa_t gpa = ghc->gpa + offset;
 
-	BUG_ON(len + offset > ghc->len);
+	if (WARN_ON_ONCE(len + offset > ghc->len))
+		return -EINVAL;
 
 	if (slots->generation != ghc->generation) {
 		if (__kvm_gfn_to_hva_cache_init(slots, ghc, ghc->gpa, ghc->len))