perf lock contention: Revise needs_callstack() condition

It needs callstacks for two reasons:

 * for stack aggregation mode, the map key is the stack id and it can
   also show the full stack traces when -v is used

 * for other aggregation modes, the stack filter can be used to limit
   lock contentions from known call paths

The -v option is meaningful (in terms of stack trace) only for stack
aggregation mode, so it should not set the save_callstack for other
mode like with -t or -l options.

I've noticed this with the following command line:

  $ sudo ./perf lock con -ablv -E 3 -M 16 -- ./perf bench sched messaging
  ...
   contended   total wait     max wait     avg wait            address   symbol

          88      4.59 ms    108.07 us     52.13 us   ffff935757f46ec0    (spinlock)
          33    905.22 us     73.67 us     27.43 us   ffff935757f41700    (spinlock)
          28    703.69 us     79.28 us     25.13 us   ffff938a3d9b0c80   rq_lock (spinlock)

  === output for debug ===

  bad: 12272, total: 12421
  bad rate: 98.80 %
  histogram of failure reasons
         task: 8285
        stack: 3987    <---------- here
         time: 0
         data: 0

It should not have any failure on stacks since it doesn't use it.
No functional change intended.

Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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