[JFFS2] Track parent inode for directories (for NFS export)

[JFFS2] Track parent inode for directories (for NFS export)

To support NFS export, we need to know the parent inode of directories.
Rather than growing the jffs2_inode_cache structure, share space with
the nlink field -- which was always set to 1 for directories anyway.

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