[JFFS2] Discard remaining free space when filing a dirty block in scan.

The incoming ref_totlen() calculation is going to rely on the existence
of nodes which cover all dirty space. We can't just tweak the accounting
data any more; we have to call jffs2_scan_dirty_space() to do it.

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