perf record: Force enable --timestamp-filename when --switch-output is provided

Without this patch, the last output doesn't have timestamp appended if
--timestamp-filename is not explicitly provided. For example:

# perf record -a --switch-output &
[1] 11224
# kill -s SIGUSR2 11224
[ perf record: dump data: Woken up 1 times ]
# [ perf record: Dump perf.data.2015122622372823 ]

# fg
perf record -a --switch-output
^C[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.027 MB perf.data (540 samples) ]

# ls -l
total 836
-rw------- 1 root root 33256 Dec 26 22:37 perf.data <---- *Odd*
-rw------- 1 root root 817156 Dec 26 22:37 perf.data.2015122622372823

Signed-off-by: Wang Nan <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Zefan Li <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: He Kuang <[e-mail]>
[ Updated man page, that also got an entry for --timestamp-filename ]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-record.txt

···
 --all-user::
 Configure all used events to run in user space.
 
+--timestamp-filename
+Append timestamp to output file name.
+
 --switch-output::
 Generate multiple perf.data files, timestamp prefixed, switching to a new one
 when receiving a SIGUSR2.
···
 A possible use case is to, given an external event, slice the perf.data file
 that gets then processed, possibly via a perf script, to decide if that
 particular perf.data snapshot should be kept or not.
+
+Implies --timestamp-filename.
 
 SEE ALSO
 --------

tools/perf/builtin-record.c

···
 		return -EINVAL;
 	}
 
+	if (rec->switch_output)
+		rec->timestamp_filename = true;
+
 	if (!rec->itr) {
 		rec->itr = auxtrace_record__init(rec->evlist, &err);
 		if (err)