We are done with the scattergather entry when the walk offset goes
past sg->offset + sg->length, not when it crosses a page boundary.

There is a similarly queer test in the second half of
scatterwalk_pagedone() that probably needs some scrutiny.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>