mtd: introduce mtd_block_isbad interface

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/ubi/io.c b/drivers/mtd/ubi/io.c
index 8d832fc..a1b683a 100644
--- a/drivers/mtd/ubi/io.c
+++ b/drivers/mtd/ubi/io.c
@@ -634,7 +634,7 @@
 	if (ubi->bad_allowed) {
 		int ret;
 
-		ret = mtd->block_isbad(mtd, (loff_t)pnum * ubi->peb_size);
+		ret = mtd_block_isbad(mtd, (loff_t)pnum * ubi->peb_size);
 		if (ret < 0)
 			ubi_err("error %d while checking if PEB %d is bad",
 				ret, pnum);