crypto: hash - Fix handling of sg entry that crosses page boundary

A quirk that we've always supported is having an sg entry that's
bigger than a page, or more generally an sg entry that crosses
page boundaries.  Even though it would be better to explicitly have
to sg entries for this, we need to support it for the existing users,
in particular, IPsec.

The new ahash sg walking code did try to handle this, but there was
a bug where we didn't increment the page so kept on walking on the
first page over an dover again.

This patch fixes it.

Tested-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>