perf tools: Fix mode setting in copyfile_mode_ns()

slow_copyfile() opens the file by name, so "write" permissions must not
be removed in copyfile_mode_ns() before calling slow_copyfile().

Example:

Before:

$ sudo chmod +r /proc/kcore
$ sudo setcap "cap_sys_admin,cap_sys_ptrace,cap_syslog,cap_sys_rawio=ep" tools/perf/perf
$ tools/perf/perf buildid-cache -k /proc/kcore
Couldn't add /proc/kcore

After:

$ sudo chmod +r /proc/kcore
$ sudo setcap "cap_sys_admin,cap_sys_ptrace,cap_syslog,cap_sys_rawio=ep" tools/perf/perf
$ tools/perf/perf buildid-cache -v -k /proc/kcore
kcore added to build-id cache directory /home/ahunter/.debug/[kernel.kcore]/37e340b1b5a7cf4f57ba8de2bc777359588a957f/2019100709562289

Signed-off-by: Adrian Hunter <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Link: http://lore.kernel.org/lkml/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/copyfile.c

···
 	if (tofd < 0)
 		goto out;
 
-	if (fchmod(tofd, mode))
-		goto out_close_to;
-
 	if (st.st_size == 0) { /* /proc? do it slowly... */
 		err = slow_copyfile(from, tmp, nsi);
+		if (!err && fchmod(tofd, mode))
+			err = -1;
 		goto out_close_to;
 	}
+
+	if (fchmod(tofd, mode))
+		goto out_close_to;
 
 	nsinfo__mountns_enter(nsi, &nsc);
 	fromfd = open(from, O_RDONLY);