perf probe: Fix short file name probe location reporting

After adding probes, perf-probe(1) reports the probes locations which include
filenames for certain cases.

But for short file names (whose length < 32), perf-probe didn't display the
name correctly. It actually skipped the first character.

Here's an example where 'icmp.c' was screwed:

   $ perf probe -n -a "icmp.c;sk=*"
   Add new events:
     probe:icmp_push_reply (on @cmp.c)
     probe:icmp_reply     (on @cmp.c)
     probe:icmp_reply_1   (on @cmp.c)
     probe:icmp_send      (on @cmp.c)
     probe:icmp_send_1    (on @cmp.c)
     probe:icmp_error     (on @cmp.c)
     probe:icmp_error_1   (on @cmp.c)
     probe:icmp_error_2   (on @cmp.c)
     probe:icmp_error_3   (on @cmp.c)

This patch fixes this bug in synthesize_perf_probe_point().
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
LKML-Reference: <m31v588r9k.fsf@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -1068,13 +1068,13 @@ static char *synthesize_perf_probe_point(struct perf_probe_point *pp)
 			goto error;
 	}
 	if (pp->file) {
-		len = strlen(pp->file) - 31;
-		if (len < 0)
-			len = 0;
-		tmp = strchr(pp->file + len, '/');
-		if (!tmp)
-			tmp = pp->file + len;
-		ret = e_snprintf(file, 32, "@%s", tmp + 1);
+		tmp = pp->file;
+		len = strlen(tmp);
+		if (len > 30) {
+			tmp = strchr(pp->file + len - 30, '/');
+			tmp = tmp ? tmp + 1 : pp->file + len - 30;
+		}
+		ret = e_snprintf(file, 32, "@%s", tmp);
 		if (ret <= 0)
 			goto error;
 	}