mtd: nand_bbt: fix theoretical integer overflow in BBT write

This statement was written with a cast-to-loff_t to be sure to have a
full 64-bit mask. However, we don't account for the fact that
'1 << this->bbt_erase_shift' might already overflow.

This will not be a problem in practice, since eraseblocks should never
be anywhere near 4GiB. But we can do this for completeness, and quiet
Coverity in the meantime. CID #1226806.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/nand_bbt.c

@@ -719,7 +719,7 @@ static int write_bbt(struct mtd_info *mtd, uint8_t *buf,
 		/* Must we save the block contents? */
 		if (td->options & NAND_BBT_SAVECONTENT) {
 			/* Make it block aligned */
-			to &= ~((loff_t)((1 << this->bbt_erase_shift) - 1));
+			to &= ~(((loff_t)1 << this->bbt_erase_shift) - 1);
 			len = 1 << this->bbt_erase_shift;
 			res = mtd_read(mtd, to, len, &retlen, buf);
 			if (res < 0) {