[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>