locking/osq: Simplify osq_unlock()

It is now easy to observe that the start of osq_unlock() and the body of
osq_wait_unlink_next() are in fact identical and there is no point in repeating
this code.

Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>