KVM: Convert comment into an assertion in kvm_io_bus_register_dev()

Instead of having a comment indicating the need to hold slots_lock
when calling kvm_io_bus_register_dev(), make it explicit with
a lockdep assertion.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231207151201.3028710-6-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

virt/kvm/kvm_main.c

@@ -5545,7 +5545,6 @@ int kvm_io_bus_read(struct kvm_vcpu *vcpu, enum kvm_bus bus_idx, gpa_t addr,
 	return r < 0 ? r : 0;
 }
 
-/* Caller must hold slots_lock. */
 int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 			    int len, struct kvm_io_device *dev)
 {
@@ -5553,6 +5552,8 @@ int kvm_io_bus_register_dev(struct kvm *kvm, enum kvm_bus bus_idx, gpa_t addr,
 	struct kvm_io_bus *new_bus, *bus;
 	struct kvm_io_range range;
 
+	lockdep_assert_held(&kvm->slots_lock);
+
 	bus = kvm_get_bus(kvm, bus_idx);
 	if (!bus)
 		return -ENOMEM;