perf/core improvements and fixes:

User visible:

- Allow generate timestamped suffixed multiple perf.data files upon receiving
  SIGUSR2 in 'perf record', to slice a long running monitoring session, allowing
  to dump uninteresting sessions (Wang Nan)

- Handle ENOMEM for perf_event_max_stack + PERF_SAMPLE_CALLCHAIN
  in perf_evsel__open_strerror(), showing a more informative
  message when the request call stack depth can't be allocated by
  the kernel (Arnaldo Carvalho de Melo)

Infrastructure:

- Use strbuf for making strings in 'perf probe' (Masami Hiramatsu)

- Do not use sizeof on pointer type, not a problem since its a pointer to
  pointer, fix none the less. Found by Coccinelle (Vaishali Thakkar)

Cleanups:

- Fix for Coverity found issues in the bpf feature build test (Florian Fainelli)

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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