[JFFS2] Relax threshold for triggering GC due to dirty blocks.

Instead of matching resv_blocks_gcmerge, which is only about 3, instead
match resv_blocks_gctrigger, which includes a proportion of the total
device size.

These ought to become tunable from userspace, at some point.

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