tty: Only perform flip buffer flush from tty_buffer_flush()

Now that dropping the buffer lock is not necessary (as result of
converting the spin lock to a mutex), the flip buffer flush no
longer needs to be handled by the buffer work.

Simply signal a flush is required; the buffer work will exit the
i/o loop, which allows tty_buffer_flush() to proceed.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>