[JFFS2] Fix dataflash support

- assume wbuf may be of size which is not power of 2
- don't make strange assumption about not padding wbuf for DataFlash
- use wbuf = DataFlash page and eraseblock >= 8 Dataflash pages

From: Peter Menzebach <pm-mtd@mw-itcon.de>
Acked-by: Artem B. Bityutskiy <dedekind@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>