KVM: Move WARN on invalid memslot index to update_memslots()

Since kvm_memslot_move_forward() can theoretically return a negative
memslot index even when kvm_memslot_move_backward() returned a positive one
(and so did not WARN) let's just move the warning to the common code.
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <eeed890ccb951e7b0dce15bc170eb2661d5b02da.1638817640.git.maciej.szmigiero@oracle.com>

virt/kvm/kvm_main.c

@@ -1322,8 +1322,7 @@ static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
 	struct kvm_memory_slot *mslots = slots->memslots;
 	int i;
 
-	if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
-	    WARN_ON_ONCE(!slots->used_slots))
+	if (slots->id_to_index[memslot->id] == -1 || !slots->used_slots)
 		return -1;
 
 	/*
@@ -1427,6 +1426,9 @@ static void update_memslots(struct kvm_memslots *slots,
 			i = kvm_memslot_move_backward(slots, memslot);
 		i = kvm_memslot_move_forward(slots, memslot, i);
 
+		if (WARN_ON_ONCE(i < 0))
+			return;
+
 		/*
 		 * Copy the memslot to its new position in memslots and update
 		 * its index accordingly.