s390/checksum: remove memset() within csum_partial_copy_from_user()

The memset() within csum_partial_copy_from_user() is rather pointless since
copy_from_user() already cleared the rest of the destination buffer if an
exception happened.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>