TTY: tty_io, annotate locking functions

tty_write_lock and tty_write_unlock contain imbalanced locking. But
this is intentional, so mark them appropriately by
__acquires/__releases.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>