perf bpf: Add kprobe example to catch 5s naps

perf bpf: Add kprobe example to catch 5s naps

Description:

. Disable strace like syscall tracing (--no-syscalls), or try tracing
  just some (-e *sleep).

. Attach a filter function to a kernel function, returning when it should
  be considered, i.e. appear on the output:

  $ cat tools/perf/examples/bpf/5sec.c
  #include <bpf.h>

  SEC("func=hrtimer_nanosleep rqtp->tv_sec")
  int func(void *ctx, int err, long sec)
  {
	  return sec == 5;
  }

  char _license[] SEC("license") = "GPL";
  int _version SEC("version") = LINUX_VERSION_CODE;
  $

. Run it system wide, so that any sleep of >= 5 seconds and < than 6
  seconds gets caught.

. Ask for callgraphs using DWARF info, so that userspace can be unwound

. While this is running, run something like "sleep 5s".

  # perf trace --no-syscalls -e tools/perf/examples/bpf/5sec.c/call-graph=dwarf/
     0.000 perf_bpf_probe:func:(ffffffff9811b5f0) tv_sec=5
                                       hrtimer_nanosleep ([kernel.kallsyms])
                                       __x64_sys_nanosleep ([kernel.kallsyms])
                                       do_syscall_64 ([kernel.kallsyms])
                                       entry_SYSCALL_64 ([kernel.kallsyms])
                                       __GI___nanosleep (/usr/lib64/libc-2.26.so)
                                       rpl_nanosleep (/usr/bin/sleep)
                                       xnanosleep (/usr/bin/sleep)
                                       main (/usr/bin/sleep)
                                       __libc_start_main (/usr/lib64/libc-2.26.so)
                                       _start (/usr/bin/sleep)
  ^C#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-2nmxth2l2h09f9gy85lyexcq@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>