UBI: block: Add missing cache flushes

commit 98fb1ffd upstream.

Block drivers are responsible for calling flush_dcache_page() on each
BIO request. This operation keeps the I$ coherent with the D$ on
architectures that don't have hardware coherency support. Without this
flush, random crashes are seen when executing user programs from an ext4
filesystem backed by a ubiblock device.

This patch is based on the change implemented in commit 2d4dc890
("block: add helpers to run flush_dcache_page() against a bio and a
request's pages").

Fixes: 9d54c8a3 ("UBI: R/O block driver on top of UBI volumes")
Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
[ luis: backported to 3.16: used Kevin's backport to 3.14 ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/mtd/ubi/block.c

@@ -255,6 +255,7 @@ static int do_ubiblock_request(struct ubiblock *dev, struct request *req)
 	mutex_lock(&dev->dev_mutex);
 	ret = ubiblock_read(dev, bio_data(req->bio), sec, len);
 	mutex_unlock(&dev->dev_mutex);
+	rq_flush_dcache_pages(req);
 
 	return ret;
 }