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 <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1401892622-30848-7-git-send-email-jolsa@kernel.orgSigned-off-by: Jiri Olsa <jolsa@kernel.org>