mtd: tests: fix more potential integer overflows

Caught by Coverity (CID #200625 and others)

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
diff --git a/drivers/mtd/tests/oobtest.c b/drivers/mtd/tests/oobtest.c
index 5e06118..64390ab 100644
--- a/drivers/mtd/tests/oobtest.c
+++ b/drivers/mtd/tests/oobtest.c
@@ -70,7 +70,7 @@
 	int i;
 	struct mtd_oob_ops ops;
 	int err = 0;
-	loff_t addr = ebnum * mtd->erasesize;
+	loff_t addr = (loff_t)ebnum * mtd->erasesize;
 
 	prandom_bytes_state(&rnd_state, writebuf, use_len_max * pgcnt);
 	for (i = 0; i < pgcnt; ++i, addr += mtd->writesize) {