Simplify TTY sending code and locking

Simplify TTY sending code and locking

The great get_elm() dance was unnecessary, since the TTY layer already
synchronises *all* calls to ldisc_ops with tty_ldisc_ref() and
tty_ldisc_lock().

All that we have to ensure is that the worker does not race against
close(), which is trivial if we clear TTY_DO_WRITE_WAKEUP and then flush
the worker.

Also, the worker no longer checks whether the netdev is up - all it
needs to do is to write out the TTY buffer, and wake up the netdev when
the buffer is empty.