jffs2: Use SLAB_HWCACHE_ALIGN for jffs2_raw_{dirent,inode} slabs

We may end up doing DMA to/from these. Until the new MTD API fixes the
issues, this should stop things from falling over.

Original idea from Gilles Casse <list@gcasse.net>

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>