[JFFS2] Trigger garbage collection when very_dirty_list size becomes excessive

[JFFS2] Trigger garbage collection when very_dirty_list size becomes excessive

With huge amounts of free space, we weren't bothering to GC for while a
while, and pathological numbers of obsolete nodes were accumulating,
seriously affecting performance on NAND flash (OLPC trac #3978)

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