KVM: Discard zero mask with function kvm_dirty_ring_reset

Function kvm_reset_dirty_gfn may be called with parameters cur_slot /
cur_offset / mask are all zero, it does not represent real dirty page.
It is not necessary to clear dirty page in this condition. Also return
value of macro __fls() is undefined if mask is zero which is called in
funciton kvm_reset_dirty_gfn(). Here just return.
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Message-ID: <20240613122803.1031511-1-maobibo@loongson.cn>
[Move the conditional inside kvm_reset_dirty_gfn; suggested by
 Sean Christopherson. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/dirty_ring.c

@@ -55,6 +55,9 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
 	struct kvm_memory_slot *memslot;
 	int as_id, id;
 
+	if (!mask)
+		return;
+
 	as_id = slot >> 16;
 	id = (u16)slot;