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>