[MTD] mtdchar.c: ioctl always returns 0 as size written for ppc64

"include/linux/mtd/mtd.h" declares "mtd_oob_ops.retlen" as size_t, which
is 64 bits on targets with a 64 bit addressing. The MEMWRITEOOB ioctl
calls copy_to_user() to write it back to "mtd_oob_buf.length", which is
declared in "include/linux/mtd-abi.h" as uint32_t. Since powerpc is a
big endian architecture, this only copies the upper 32 bits of the
address, which is always 0.
Signed-off-by: David Scidmore <dscidmore@xes-inc.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

drivers/mtd/mtdchar.c

@@ -483,6 +483,7 @@ static int mtd_ioctl(struct inode *inode, struct file *file,
 	{
 		struct mtd_oob_buf buf;
 		struct mtd_oob_ops ops;
+	        uint32_t retlen;
 
 		if(!(file->f_mode & 2))
 			return -EPERM;
@@ -522,8 +523,11 @@ static int mtd_ioctl(struct inode *inode, struct file *file,
 		buf.start &= ~(mtd->oobsize - 1);
 		ret = mtd->write_oob(mtd, buf.start, &ops);
 
-		if (copy_to_user(argp + sizeof(uint32_t), &ops.oobretlen,
-				 sizeof(uint32_t)))
+		if (ops.oobretlen > 0xFFFFFFFFU)
+			ret = -EOVERFLOW;
+		retlen = ops.oobretlen;
+		if (copy_to_user(&((struct mtd_oob_buf *)argp)->length,
+				 &retlen, sizeof(buf.length)))
 			ret = -EFAULT;
 
 		kfree(ops.oobbuf);