[JFFS2] Calculate CRC check starting point correctly

When data starts from the beginning of NAND page, 'len' must be zero, not
c->wbuf_page.

Thanks to Zoltan Sogor for reporting this problem.

Signed-off-by: Artem B. Bityutskiy <dedekind@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>