tty: flush flip buffer on ldisc input queue flush

Flush the tty flip buffer when the line discipline input queue is flushed,
including the user call tcflush(TCIFLUSH/TCIOFLUSH).  This prevents
unexpected stale data after a user application calls tcflush().

Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Antonino Ingargiola <tritemio@gmail.com>
Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>