[JFFS2] Locking issues in summary write code.

We can't use jffs2_scan_dirty_space() because it doesn't do any locking; it's
only for use at scan time -- hence the 'scan' in the name.

Also, don't allocate refs while we have c->erase_completion_lock held.

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