perf trace: Disable events and drain events when forked workload ends

We were not checking in the inner event processing loop if the forked workload
had finished, which, on a busy system, may make it take a long time trying to
drain events, entering a seemingly neverending loop, waiting for the system to
get idle enough to make it drain the buffers.

Fix it by disabling the events when 'done' is true, in the inner loop, to start
draining what is in the buffers.

Now:

[root@ssdandy ~]# time trace --filter-pids 14003 -a sleep 1 | tail
  996.748 ( 0.002 ms): sh/30296 rt_sigprocmask(how: SETMASK, nset: 0x7ffc83418160, sigsetsize: 8) = 0
  996.751 ( 0.002 ms): sh/30296 rt_sigprocmask(how: BLOCK, nset: 0x7ffc834181f0, oset: 0x7ffc83418270, sigsetsize: 8) = 0
  996.755 ( 0.002 ms): sh/30296 rt_sigaction(sig: INT, act: 0x7ffc83417f50, oact: 0x7ffc83417ff0, sigsetsize: 8) = 0
 1004.543 ( 0.362 ms): tail/30198  ... [continued]: read()) = 4096
 1004.548 ( 7.791 ms): sh/30296 wait4(upid: -1, stat_addr: 0x7ffc834181a0) ...
 1004.975 ( 0.427 ms): tail/30198 read(buf: 0x7633f0, count: 8192) = 4096
 1005.390 ( 0.410 ms): tail/30198 read(buf: 0x765410, count: 8192) = 4096
 1005.743 ( 0.348 ms): tail/30198 read(buf: 0x7633f0, count: 8192) = 4096
 1006.197 ( 0.449 ms): tail/30198 read(buf: 0x765410, count: 8192) = 4096
 1006.492 ( 0.290 ms): tail/30198 read(buf: 0x7633f0, count: 8192) = 4096

real	0m1.219s
user	0m0.704s
sys	0m0.331s
[root@ssdandy ~]#

Reported-by: Michael Petlan <mpetlan@redhat.com>
Suggested-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
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: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-p6kpn1b26qcbe47pufpw0tex@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-trace.c

@@ -2273,6 +2273,11 @@ next_event:
 
 			if (interrupted)
 				goto out_disable;
+
+			if (done && !draining) {
+				perf_evlist__disable(evlist);
+				draining = true;
+			}
 		}
 	}