perf trace: Read thread's COMM from /proc when not set

We get notifications for threads that gets created while we're tracing,
but for preexisting threads we may end not having synthesized them, like
when tracing a 'perf trace' session that will use '--pid' to trace some
other thread.

And besides we should probably stop synthesizing those records and
instead read thread information in a lazy way, i.e. just when we need,
like done in this patch:

Now the 'pid_t' argument in 'perf_event_open' gets translated to a COMM:

  # perf trace -e perf_event_open perf stat -e cycles -p 31601
     0.027 ( 0.027 ms): perf/23393 perf_event_open(attr_uptr: 0x2fdd0d8, pid: 31601 (abrt-dump-journ), cpu: -1, group_fd: -1, flags: FD_CLOEXEC)
                                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= 3
^C

And in other syscalls containing pid_t without thread->comm_set at the
time of the formatting.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-ioeps6dlwst17d6oozc9shtk@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/trace/beauty/pid.c

@@ -3,9 +3,12 @@ static size_t syscall_arg__scnprintf_pid(char *bf, size_t size, struct syscall_a
 	int pid = arg->val;
 	struct trace *trace = arg->trace;
 	size_t printed = scnprintf(bf, size, "%d", pid);
-	struct thread *thread = machine__find_thread(trace->host, pid, pid);
+	struct thread *thread = machine__findnew_thread(trace->host, pid, pid);
 
 	if (thread != NULL) {
+		if (!thread->comm_set)
+			thread__set_comm_from_proc(thread);
+
 		if (thread->comm_set)
 			printed += scnprintf(bf + printed, size - printed,
 					     " (%s)", thread__comm_str(thread));