wait.[ch]: Introduce the simple waitqueue (swait) implementation

The existing wait queue support has support for custom wake up call
backs, wake flags, wake key (passed to call back) and exclusive
flags that allow wakers to be tagged as exclusive, for limiting
the number of wakers.

In a lot of cases, none of these features are used, and hence we
can benefit from a slimmed down version that lowers memory overhead
and reduces runtime overhead.

The concept originated from -rt, where waitqueues are a constant
source of trouble, as we can't convert the head lock to a raw
spinlock due to fancy and long lasting callbacks.

With the removal of custom callbacks, we can use a raw lock for
queue list manipulations, hence allowing the simple wait support
to be used in -rt.

[Patch is from PeterZ which is based on Thomas version. Commit message is
 written by Paul G.
 Daniel:  - Fixed some compile issues
 	  - Added non-lazy implementation of swake_up_locked as suggested
	     by Boqun Feng.]

Originally-by: Thomas Gleixner <[email protected]>
Signed-off-by: Daniel Wagner <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: [email protected]
Cc: Boqun Feng <[email protected]>
Cc: Marcelo Tosatti <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

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