locking/ww_mutex: Notify waiters that have to back off while adding tasks to wait list

While adding our task as a waiter, detect if another task should back off
because of us.

With this patch, we establish the invariant that the wait list contains
at most one (sleeping) waiter with ww_ctx->acquired > 0, and this waiter
will be the first waiter with a context.

Since only waiters with ww_ctx->acquired > 0 have to back off, this allows
us to be much more economical with wakeups.

Signed-off-by: Nicolai Hähnle <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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