rb_tree: fix stable/13 linux/rbtree.h post ABI fix

Rearranging the left, right, and parent pointers for backward
compatibility with stable/13 in
d8a88ec381498f5942403088d28ee325b92e9a78 is incomplete because it does
not change linux/rbtree.h so that it too knows how rb_left and
rb_right are redefined. So complete it.

Reported by: jon_xyinn.org, jgibbons@protogate.org
Tested by: manu, jon_xyinn.org
Differential Revision: https://reviews.freebsd.org/D37743