rbtree: avoid generating code twice for the cached versions (tools copy)

As was already noted in rbtree.h, the logic to cache rb_first (or
rb_last) can easily be implemented externally to the core rbtree api.

This commit takes the changes applied to the include/linux/ and lib/
rbtree files in 9f973cb3 ("lib/rbtree: avoid generating code twice
for the cached versions"), and applies these to the
tools/include/linux/ and tools/lib/ files as well to keep them
synchronized.

Link: http://lkml.kernel.org/r/20190703034812.53002-1-walken@google.com


Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>