UBI: fix backward compatibility

Commit 32bc4820287a1a03982979515949e8ea56eac641 did not fully fix
the backward compatibility issues. We still fail to properly handle
situations when the first PEB contains non-zero image sequence
number, but one of the following PEBs contains zero image sequence
number. For example, this may happen if we mount a new image with
an old kernel, and then try to mount it in the new kernel.

This patch should fix the issue.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/scan.c

@@ -794,16 +794,15 @@ static int process_eb(struct ubi_device *ubi, struct ubi_scan_info *si,
 		 * number.
 		 */
 		image_seq = be32_to_cpu(ech->image_seq);
-		if (!si->image_seq_set) {
+		if (!ubi->image_seq && image_seq)
 			ubi->image_seq = image_seq;
-			si->image_seq_set = 1;
-		} else if (ubi->image_seq && ubi->image_seq != image_seq) {
+		if (ubi->image_seq && image_seq &&
+		    ubi->image_seq != image_seq) {
 			ubi_err("bad image sequence number %d in PEB %d, "
 				"expected %d", image_seq, pnum, ubi->image_seq);
 			ubi_dbg_dump_ec_hdr(ech);
 			return -EINVAL;
 		}
-
 	}
 
 	/* OK, we've done with the EC header, let's look at the VID header */

drivers/mtd/ubi/scan.h

@@ -103,7 +103,6 @@ struct ubi_scan_volume {
  * @ec_sum: a temporary variable used when calculating @mean_ec
  * @ec_count: a temporary variable used when calculating @mean_ec
  * @corr_count: count of corrupted PEBs
- * @image_seq_set: indicates @ubi->image_seq is known
  *
  * This data structure contains the result of scanning and may be used by other
  * UBI sub-systems to build final UBI data structures, further error-recovery
@@ -127,7 +126,6 @@ struct ubi_scan_info {
 	uint64_t ec_sum;
 	int ec_count;
 	int corr_count;
-	int image_seq_set;
 };
 
 struct ubi_device;