perf tools: Cache dso data file descriptor

Caching dso data file descriptors to avoid expensive re-opens
especially during DWARF unwind.

We keep dsos data file descriptors open until their count reaches
the half of the current fd open limit (RLIMIT_NOFILE). In this case
we close file descriptor of the first opened dso object.

We've got overall speedup (~27% for my workload) of report:
 'perf report --stdio -i perf-test.data' (3 runs)
  (perf-test.data size was around 12GB)

  current code:
   545,640,944,228      cycles                     ( +-  0.53% )
   785,255,798,320      instructions               ( +-  0.03% )

     366.340910010 seconds time elapsed            ( +-  3.65% )

  after change:
   435,895,036,114      cycles                     ( +-  0.26% )
   636,790,271,176      instructions               ( +-  0.04% )

     266.481463387 seconds time elapsed            ( +-  0.13% )

Acked-by: Namhyung Kim <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Corey Ashford <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jean Pihet <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jiri Olsa <[email protected]>

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