tty: Termios locking - sort out real_tty confusions and lock reads

commit 8f520021 upstream

(only the tty_io.c portion of this commit)

This moves us towards sanity and should mean our termios locking is now
complete and comprehensive.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>