s390/checksum: provide csum_partial_copy_nocheck()

With csum_partial(), which reads all bytes into registers it is easy to
also implement csum_partial_copy_nocheck() which copies the buffer while
calculating its checksum.

For a 512 byte buffer this reduces the runtime by 19%. Compared to the old
generic variant (memcpy() + cksm instruction) runtime is reduced by 42%).

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>