Add a wakeup call for a case whereby the caller already has the waitqueue
spinlock held.  This can be used by pipes to alter the ring buffer indices
and issue a wakeup under the same spinlock.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>