UBI: block: Add missing cache flushes

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 2d4dc89
("block: add helpers to run flush_dcache_page() against a bio and a
request's pages").

Fixes: 9d54c8a ("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>

drivers/mtd/ubi/block.c

@@ -310,6 +310,8 @@ static void ubiblock_do_work(struct work_struct *work)
 	blk_rq_map_sg(req->q, req, pdu->usgl.sg);
 
 	ret = ubiblock_read(pdu);
+	rq_flush_dcache_pages(req);
+
 	blk_mq_end_request(req, ret);
 }