rb_tree: reorganize code in rb_erase() for additional changes

rb_tree: reorganize code in rb_erase() for additional changes

First, move some code around in order to make the next change more obvious.

[akpm@linux-foundation.org: coding-style fixes]
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 16c047add3ceaf0ab882e3e094d1ec904d02312d]

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>