perf trace: Print thread info when following children

The default for 'trace workload' is to set perf_event_attr.inherit to 1,
i.e. to make it equivalent to 'strace -f workload', so we were ending
with syscalls for multiple processes mixed up, fix it:

Before:

  [root@ssdandy ~]# trace -e brk time usleep 1
     0.071 ( 0.002 ms): brk(              ) = 0x100e000
     0.802 ( 0.001 ms): brk(              ) = 0x1d99000
     1.132 ( 0.003 ms): brk(              ) = 0x1d99000
     1.136 ( 0.003 ms): brk(brk: 0x1dba000) = 0x1dba000
     1.140 ( 0.001 ms): brk(              ) = 0x1dba000
  0.00user 0.00system 0:00.00elapsed 63%CPU (0avgtext+0avgdata 528maxresident)k
  0inputs+0outputs (0major+181minor)pagefaults 0swaps
  [root@ssdandy ~]#

After:

  [root@ssdandy ~]# trace -f -e brk time usleep 1
     0.072 ( 0.002 ms): time/26308 brk(               ) = 0x1e6e000
     0.860 ( 0.001 ms): usleep/26309 brk(             ) = 0xb91000
     1.193 ( 0.003 ms): usleep/26309 brk(             ) = 0xb91000
     1.197 ( 0.003 ms): usleep/26309 brk(brk: 0xbb2000) = 0xbb2000
     1.201 ( 0.001 ms): usleep/26309 brk(             ) = 0xbb2000
  0.00user 0.00system 0:00.00elapsed 0%CPU (0avgtext+0avgdata 524maxresident)k
  0inputs+0outputs (0major+180minor)pagefaults 0swaps
  [root@ssdandy ~]#

BTW: to achieve the 'strace workload' behaviour, i.e. without a explicit
'-f', one has to use --no-inherit.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
echo Link: http://lkml.kernel.org/n/tip-`ranpwd -l 24`@git.kernel.org
Link: http://lkml.kernel.org/n/tip-2wu2d5n65msxoq1i7vtcaft2@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-trace.c

@@ -2114,7 +2114,9 @@ static int trace__run(struct trace *trace, int argc, const char **argv)
 	else
 		perf_evlist__enable(evlist);
 
-	trace->multiple_threads = evlist->threads->map[0] == -1 || evlist->threads->nr > 1;
+	trace->multiple_threads = evlist->threads->map[0] == -1 ||
+				  evlist->threads->nr > 1 ||
+				  perf_evlist__first(evlist)->attr.inherit;
 again:
 	before = trace->nr_events;