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

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>
[Linux commit 4c60117811171d867d4f27f17ea07d7419d45dae]

Ported to Xen.

Signed-off-by: Praveen Kumar <kpraveen.lkml@gmail.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>