rb_tree: make clear distinction between two different cases in rb_erase()

There are two cases when a node, having 2 childs, is erased:
'normal case': the successor is not the right-hand-child of the node to be erased
'special case': the successor is the right-hand child of the node to be erased

Here some ascii-art, with following symbols (referring to the code):
O: node to be deleted
N: the successor of O
P: parent of N
C: child of N
L: some other node

normal case:

               O                         N
              / \                       / \
             /   \                     /   \
            L     \                   L     \
           / \     P      ---->      / \     P
                  / \                       / \
                 /                         /
                N                         C
                 \                       / \
                  \
                   C
                  / \

special case:
              O|P                        N
              / \                       / \
             /   \                     /   \
            L     \                   L     \
           / \     N      ---->      /       C
                    \                       / \
                     \
                      C
                     / \

Notice that for the special case we don't have to reconnect C to N.

Signed-off-by: Wolfram Strepp <wstrepp@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/rbtree.c

@@ -244,13 +244,13 @@ void rb_erase(struct rb_node *node, struct rb_root *root)
 		parent = rb_parent(node);
 		color = rb_color(node);
 
-		if (child)
-			rb_set_parent(child, parent);
 		if (parent == old) {
-			parent->rb_right = child;
 			parent = node;
-		} else
+		} else {
+			if (child)
+				rb_set_parent(child, parent);
 			parent->rb_left = child;
+		}
 
 		node->rb_parent_color = old->rb_parent_color;
 		node->rb_right = old->rb_right;