mtd: partitions: redboot: Added conversion of operands to a larger type

The value of an arithmetic expression directory * master->erasesize is
subject to overflow due to a failure to cast operands to a larger data
type before perfroming arithmetic

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Denis Arefev <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

drivers/mtd/parsers/redboot.c

···
 			offset -= master->erasesize;
 		}
 	} else {
-		offset = directory * master->erasesize;
+		offset = (unsigned long) directory * master->erasesize;
 		while (mtd_block_isbad(master, offset)) {
 			offset += master->erasesize;
 			if (offset == master->size)