[JFFS2] Don't advance c->wbuf_ofs to next eraseblock after wbuf flush

After flushing the last page of an eraseblock, don't leave the
wbuf 'offset' field pointing at the start of the next physical
eraseblock. This was causing a BUG() on NOR-ECC (Sibley) flash, where
we start writing a little further in, after the cleanmarker.

Debugged by Alexander Belyakov <abelyako@googlemail.com>

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