[JFFS2] Introduce jffs2_scan_dirty_space() function.

To eliminate the __totlen field from struct jffs2_raw_node_ref, we need
to allocate nodes for dirty space instead of just tweaking the accounting
data. Introduce jffs2_scan_dirty_space() in preparation for that.

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