locking/qspinlock: Extract out code snippets for the next patch

This is a preparatory patch that extracts out the following 2 code
snippets to prepare for the next performance optimization patch.

 1) the logic for the exchange of new and previous tail code words
    into a new xchg_tail() function.
 2) the logic for clearing the pending bit and setting the locked bit
    into a new clear_pending_set_locked() function.

This patch also simplifies the trylock operation before queuing by
calling queued_spin_trylock() directly.

Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Daniel J Blueman <[email protected]>
Cc: David Vrabel <[email protected]>
Cc: Douglas Hatch <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Raghavendra K T <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Scott J Norton <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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