Check whether the destination buffer is written to beyond the last
byte contained in the scatterlist.
    
Also change IDX1 of the cross-page access offsets to a multiple of 4.
This triggers a corruption in the HIFN driver and doesn't seem to
negatively impact other testcases.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>