Simplify the timerqueue code by using cached rbtrees and rely on the tree
leftmost node semantics to get the timer with earliest expiration time.
This is a drop in conversion, and therefore semantics remain untouched.

The runtime overhead of cached rbtrees is be pretty much the same as the
current head->next method, noting that when removing the leftmost node,
a common operation for the timerqueue, the rb_next(leftmost) is O(1) as
well, so the next timer will either be the right node or its parent.
Therefore no extra pointer chasing. Finally, the size of the struct
timerqueue_head remains the same.

Passes several hours of rcutorture.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190724152323.bojciei3muvfxalm@linux-r8p5