KVM: x86/mmu: Replace comment with an actual lockdep assertion on mmu_lock

Assert that mmu_lock is held for write in __walk_slot_rmaps() instead of
hoping the function comment will magically prevent introducing bugs.
Signed-off-by: Ben Gardon <bgardon@google.com>
Link: https://lore.kernel.org/r/20230202182809.1929122-3-bgardon@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -5906,7 +5906,6 @@ typedef bool (*slot_rmaps_handler) (struct kvm *kvm,
 				    struct kvm_rmap_head *rmap_head,
 				    const struct kvm_memory_slot *slot);
 
-/* The caller should hold mmu-lock before calling this function. */
 static __always_inline bool __walk_slot_rmaps(struct kvm *kvm,
 					      const struct kvm_memory_slot *slot,
 					      slot_rmaps_handler fn,
@@ -5916,6 +5915,8 @@ static __always_inline bool __walk_slot_rmaps(struct kvm *kvm,
 {
 	struct slot_rmap_walk_iterator iterator;
 
+	lockdep_assert_held_write(&kvm->mmu_lock);
+
 	for_each_slot_rmap_range(slot, start_level, end_level, start_gfn,
 			end_gfn, &iterator) {
 		if (iterator.rmap)