lib/idr.c: Fix bug introduced by RCU fix

The last patch to lib/idr.c caused a bug if idr_get_new_above() was
called on an empty idr.

Usually, nodes stay on the same layer.  New layers are added to the top
of the tree.

The exception is idr_get_new_above() on an empty tree: In this case, the
new root node is first added on layer 0, then moved upwards.  p->layer
was not updated.

As usual: You shall never rely on the source code comments, they will
only mislead you.
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/idr.c

@@ -220,8 +220,14 @@ static int idr_get_empty_slot(struct idr *idp, int starting_id,
 	 */
 	while ((layers < (MAX_LEVEL - 1)) && (id >= (1 << (layers*IDR_BITS)))) {
 		layers++;
-		if (!p->count)
+		if (!p->count) {
+			/* special case: if the tree is currently empty,
+			 * then we grow the tree by moving the top node
+			 * upwards.
+			 */
+			p->layer++;
 			continue;
+		}
 		if (!(new = get_from_free_list(idp))) {
 			/*
 			 * The allocation failed.  If we built part of