perf report: Support --header-only for pipe mode

perf report: Support --header-only for pipe mode

The --header-only checks file header and prints the feature data.  But
as pipe mode doesn't have it in the header it prints almost nothing.
Change it to process first few records until it founds HEADER_FEATURE.

Before:
  $ perf record -o- true | perf report -i- --header-only
  # ========
  # captured on    : Thu Dec 10 14:34:59 2020
  # header version : 1
  # data offset    : 0
  # data size      : 0
  # feat offset    : 0
  # ========
  #

After:
  $ perf record -o- true | perf report -i- --header-only
  # ========
  # captured on    : Thu Dec 10 14:49:11 2020
  # header version : 1
  # data offset    : 0
  # data size      : 0
  # feat offset    : 0
  # ========
  #
  # hostname : balhae
  # os release : 5.7.17-1xxx
  # perf version : 5.10.rc6.gdb0ea13cc741
  # arch : x86_64
  # nrcpus online : 8
  # nrcpus avail : 8
  # cpudesc : Intel(R) Core(TM) i7-8665U CPU @ 1.90GHz
  # cpuid : GenuineIntel,6,142,12
  # total memory : 16158916 kB
  # cmdline : perf record -o- true
  # event : name = cycles, , id = { 81, 82, 83, 84, 85, 86, 87, 88 }, size = 120, ...
  # CPU_TOPOLOGY info available, use -I to display
  # NUMA_TOPOLOGY info available, use -I to display
  # pmu mappings: intel_pt = 9, intel_bts = 8, software = 1, power = 20, uprobe = 7, ...
  # time of first sample : 0.000000
  # time of last sample : 0.000000
  # sample duration :      0.000 ms
  # MEM_TOPOLOGY info available, use -I to display
  # cpu pmu capabilities: branches=32, max_precise=3, pmu_name=skylake

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20201210061302.88213-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>