tty serialize flush_to_ldisc

Serialize processing of tty buffers in flush_to_ldisc
to fix (very rare) corruption of tty buffer free list
on SMP systems.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>