[PATCH] 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>
---
 lib/idr.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/lib/idr.c b/lib/idr.c
index 7a785a0..1c4f928 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -220,8 +220,14 @@ build_up:
 	 */
 	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
-- 
1.5.6.5

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