From: William Lee Irwin III <wli@holomorphy.com>

This patch implements wake-one semantics for buffer_head wakeups in a single
step.  The buffer_head being waited on is passed to the waiter's wakeup
function by the waker, and the wakeup function compares that to the a pointer
stored in its on-stack structure and checking the readiness of the bit there
also.  Wake-one semantics are achieved by using WQ_FLAG_EXCLUSIVE in the
codepaths waiting to acquire the bit for mutual exclusion.