tty: More detailed printing on unpaired slaves

In case of errors it provides us more information about what is going on.

For example, in my test case I now can see where the error came from

 | (00.027248) tty: Unpaired slave 0
 | (00.027261) tty: Unpaired slave 2
 | (00.027273) Error (tty.c:174): tty: Only one slave external peer is allowed (index 2)
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tty.c

@@ -166,9 +166,12 @@ int tty_verify_active_pairs(void)
 				return -1;
 			}
 
+			pr_debug("Unpaired slave %d\n", tty_get_index(i));
+
 			if (++unpaired_slaves > 1) {
 				pr_err("Only one slave external peer "
-				       "is allowed\n");
+				       "is allowed (index %d)\n",
+				       tty_get_index(i));
 				return -1;
 			}
 		}