tty: Fix tty_send_xchar() lock order inversion

tty: Fix tty_send_xchar() lock order inversion

Orabug: 22623853

commit ee0c1a65cf95230d5eb3d9de94fd2ead9a428c67 upstream.

The correct lock order is atomic_write_lock => termios_rwsem, as
established by tty_write() => n_tty_write().

Fixes: c274f6ef1c666 ("tty: Hold termios_rwsem for tcflow(TCIxxx)")
Reported-and-Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit ff4fcbdf43bc4ae760ef1e7782e233156ed90a75)
Signed-off-by: Dan Duval <dan.duval@oracle.com>