UBI: fix autoresize handling in R/O mode

Currently UBI fails in autoresize when it is in R/O mode (e.g., because the
underlying MTD device is R/O). This patch fixes the issue - we just skip
autoresize and print a warning.
Reported-by: Pali Rohár <pali.rohar@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/build.c

@@ -799,6 +799,11 @@ static int autoresize(struct ubi_device *ubi, int vol_id)
 	struct ubi_volume *vol = ubi->volumes[vol_id];
 	int err, old_reserved_pebs = vol->reserved_pebs;
 
+	if (ubi->ro_mode) {
+		ubi_warn("skip auto-resize because of R/O mode");
+		return 0;
+	}
+
 	/*
 	 * Clear the auto-resize flag in the volume in-memory copy of the
 	 * volume table, and 'ubi_resize_volume()' will propagate this change