TTY: define tty_wait_until_sent_from_close

We need this helper to fix system stalls. The issue is that the rest
of the system TTYs wait for us to finish waiting. This wasn't an issue
with BKL. BKL used to unlock implicitly.

This is based on the Arnd suggestion.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>