tty: use new tty_insert_flip_string_and_push_buffer() in pty_write()

There is a race in pty_write(). pty_write() can be called in parallel
with e.g. ioctl(TIOCSTI) or ioctl(TCXONC) which also inserts chars to
the buffer. Provided, tty_flip_buffer_push() in pty_write() is called
outside the lock, it can commit inconsistent tail. This can lead to out
of bounds writes and other issues. See the Link below.

To fix this, we have to introduce a new helper called
tty_insert_flip_string_and_push_buffer(). It does both
tty_insert_flip_string() and tty_flip_buffer_commit() under the port
lock. It also calls queue_work(), but outside the lock. See
71a174b39f10 (pty: do tty_flip_buffer_push without port->lock in
pty_write) for the reasons.

Keep the helper internal-only (in drivers' tty.h). It is not intended to
be used widely.

Link: https://seclists.org/oss-sec/2022/q2/155
Fixes: 71a174b39f10 (pty: do tty_flip_buffer_push without port->lock in pty_write)
Cc: 一只狗 <chennbnbnb@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220707082558.9250-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
