leds: Extends disk trigger for reads and writes

This adds two new disk triggers for triggering on reads
and writes respectively, named "disk-read" and "disk-write".

The use case comes from working on the D-Link DNS-313 NAS
box. This features an RGB LED for disk activity. with
these two triggers I can couple the green LED to read
activity and the red LED to write activity, which gives
the appropriate user feedback about what is happening
on the disk. When tested it gave exactly the feedback
desired.

The in-kernel interface is simply changed to pass a bool
indicating if the activity is write activity and update
each trigger (and the composite "disk-activity" trigger)
depending on what is passed in.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>