[JFFS2] Correct cleanmarker checks -- we should use only 8 bytes

Commit a7a6ace1406f95c3edb8365788f85984377f3832 revamped the OOB
handling but accidentally switched to 12-byte cleanmarkers, which is
incompatible with what 'flash_eraseall -j' will do. So using
flash_eraseall -j and then trying to mount the 'empty' flash will fail,
because the cleanmarkers aren't recognised.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

fs/jffs2/wbuf.c

@@ -1008,8 +1008,8 @@ exit:
 
 #define NR_OOB_SCAN_PAGES 4
 
-/* For historical reasons we use only 12 bytes for OOB clean marker */
-#define OOB_CM_SIZE 12
+/* For historical reasons we use only 8 bytes for OOB clean marker */
+#define OOB_CM_SIZE 8
 
 static const struct jffs2_unknown_node oob_cleanmarker =
 {