slirp: use less predictable directory name in /tmp for smb config (CVE-2015-4037)

In this version I used mkdtemp(3) which is:

        _BSD_SOURCE
        || /* Since glibc 2.10: */
            (_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)

(POSIX.1-2008), so should be available on systems we care about.

While at it, reset the resulting directory name within smb structure
on error so cleanup function wont try to remove directory which we
failed to create.
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>

net/slirp.c

@@ -481,7 +481,6 @@ static void slirp_smb_cleanup(SlirpState *s)
 static int slirp_smb(SlirpState* s, const char *exported_dir,
                      struct in_addr vserver_addr)
 {
-    static int instance;
     char smb_conf[128];
     char smb_cmdline[128];
     struct passwd *passwd;
@@ -505,10 +504,10 @@ static int slirp_smb(SlirpState* s, const char *exported_dir,
         return -1;
     }
 
-    snprintf(s->smb_dir, sizeof(s->smb_dir), "/tmp/qemu-smb.%ld-%d",
-             (long)getpid(), instance++);
-    if (mkdir(s->smb_dir, 0700) < 0) {
+    snprintf(s->smb_dir, sizeof(s->smb_dir), "/tmp/qemu-smb.XXXXXX");
+    if (!mkdtemp(s->smb_dir)) {
         error_report("could not create samba server dir '%s'", s->smb_dir);
+        s->smb_dir[0] = 0;
         return -1;
     }
     snprintf(smb_conf, sizeof(smb_conf), "%s/%s", s->smb_dir, "smb.conf");