jffs2: Add 'work_done' return value from jffs2_erase_pending_blocks()

We're about to start calling this from the jffs2_garbage_collect_pass(), and
we'll want to know whether it actually did anything or not.

Signed-off-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>