block: Add missing flush_dcache_page() call

Since a bio can point to userspace pages (e.g. direct IO), this is
generally necessary.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -994,6 +994,8 @@ void bio_copy_data_iter(struct bio *dst, struct bvec_iter *dst_iter,
 		kunmap_atomic(dst_p);
 		kunmap_atomic(src_p);
 
+		flush_dcache_page(dst_bv.bv_page);
+
 		bio_advance_iter(src, src_iter, bytes);
 		bio_advance_iter(dst, dst_iter, bytes);
 	}