tty: hold lock across tty buffer finding and buffer filling

tty_buffer_request_room is well protected, but while after it returns,
 it releases the port->lock. tty->buf.tail might be modified
by either irq handler or other threads. The patch adds more protection
by holding the lock across tty buffer finding and buffer filling.

Signed-off-by: Alek Du <alek.du@intel.com>
Signed-off-by: Xiaobing Tu <xiaobing.tu@intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>