trivial: Documentation/rbtree.txt: cleanup kerneldoc of rbtree.txt

 The first formal parameter of the rb_link_node() is a pointer, and the
 "node" is define a data struct (pls see line 67 and line 73 in the
 doc), so the actual parameter should use "&data->node".

Signed-off-by: Figo.zhang <figo.zhang@kolorific.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>