[JFFS2] Repack some on-medium structures. ARM is weirder than I thought.

We have to pack at least the jint16_t structure, because otherwise it'll
be four bytes in size. Thankfully, we can do that and _not_ pack the
actual node structures, and the compiler still doesn't emit stupid code.

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