jffs2: include <linux/mtd/compat.h> instead of defining own min_t

Include <linux/mtd/compat.h> header for min_t definition instead of
providing our own one. Removes warnings in case of OneNAND support
enabled.

Although I thinks it's a bit silly to include <linux/mtd/compat.h>
just for min_t...

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Acked-by: Stefan Roese <sr@denx.de>