locking/qspinlock_stat: Count instances of nested lock slowpaths

Queued spinlock supports up to 4 levels of lock slowpath nesting -
user context, soft IRQ, hard IRQ and NMI. However, we are not sure how
often the nesting happens.

So add 3 more per-CPU stat counters to track the number of instances where
nesting index goes to 1, 2 and 3 respectively.

On a dual-socket 64-core 128-thread Zen server, the following were the
new stat counter values under different circumstances:

         State                         slowpath   index1   index2   index3
         -----                         --------   ------   ------   -------
  After bootup                         1,012,150    82       0        0
  After parallel build + perf-top    125,195,009    82       0        0

So the chance of having more than 2 levels of nesting is extremely low.

[ mingo: Minor changelog edits. ]

Signed-off-by: Waiman Long <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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