jffs2: rewrite jffs2 scanning code based on Linux one

Rewrites jffs2_1pass_build_lists() function in style of Linux's
jffs2_scan_medium() and jffs2_scan_eraseblock().
This includes:
 - Caching flash acceses
 - Smart dealing with free space

Signed-off-by: Alexey Neyman <avn@emcraft.com>
Signed-off-by: Ilya Yanok <yanok@emcraft.com>