Port libtree commit 8cf49ec927e838675597d61b5f2eb56a2db16f05.

{avl,rb}tree_replace(): fixed swaped arguments to set_child() call

Signed-off-by: Raphael Manfredi <Raphael_Manfredi@pobox.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>