tty: Fix termios tty window resize race with pty/tty pair

Kanru Chen posted a patch versus the old code which deals with the case
where you resize the pty side of a pty/tty pair. In that situation the
termios data is updated for both pty and tty but the locks are not held
for the right side.

This implements the fix differently against the updated tty code. Patch
by self but the hard bit (noticing and fixing the bug) is thanks to Kanru
Chen.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>