kernel: mtdsplit_uimage: fix rootfs offset

The return value of the find_header function need to be added to the
uimage_size, otherwise mtd_find_rootfs_from() might search for a rootfs
within a custom header and fails.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/generic/files/drivers/mtd/mtdsplit/mtdsplit_uimage.c

@@ -126,7 +126,7 @@ static int __mtdsplit_parse_uimage(struct mtd_info *master,
 		}
 		header = (struct uimage_header *)(buf + ret);
 
-		uimage_size = sizeof(*header) + be32_to_cpu(header->ih_size);
+		uimage_size = sizeof(*header) + be32_to_cpu(header->ih_size) + ret;
 		if ((offset + uimage_size) > master->size) {
 			pr_debug("uImage exceeds MTD device \"%s\"\n",
 				 master->name);