freelist: Implement lockless freelist

A simple CAS-based lock-free free list. Not the fastest thing in the world
under heavy contention, but simple and correct (assuming nodes are never
freed until after the free list is destroyed), and fairly speedy under low
contention.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/159870622579.1229682.16729440870040944993.stgit@devnote2