tty_set_ldisc receive_room fix

Fix tty_set_ldisc in tty_io.c so that tty->receive_room is only cleared if actually changing line disciplines.

Without this fix a problem occurs when requesting the line discipline to change to the same line discipline. In this case tty->receive_room is cleared but ldisc->open() is not called to set tty->receive_room back to a sane value. The result is that tty->receive_room is stuck at 0 preventing the tty flip buffer from passing receive data to the line discipline.

For example: a switch from N_TTY to N_TTY followed by a select() call for read input results in data never being received because tty->receive_room is stuck at zero.

A switch from N_TTY to N_TTY followed by
a read() call works because the read() call itself sets tty->receive_room correctly (but select does not).

Previously (< 2.6.18) this was not a problem because the tty flip buffer pushed data to the line discipline without regard for tty->receive room.

@@ -936,13 +936,6 @@ restart:
 		return -EINVAL;
 
 	/*
-	 *	No more input please, we are switching. The new ldisc
-	 *	will update this value in the ldisc open function
-	 */
-
-	tty->receive_room = 0;
-
-	/*
 	 *	Problem: What do we do if this blocks ?
 	 */
 

@@ -953,6 +946,13 @@ restart:
 		return 0;
 	}
 
+	/*
+	 *	No more input please, we are switching. The new ldisc
+	 *	will update this value in the ldisc open function
+	 */
+
+	tty->receive_room = 0;
+
 	o_ldisc = tty->ldisc;
 	o_tty = tty->link;