s390/crypto: fix stckf loop

The store-clock-fast loop in generate_entropy() mixes (exors)
only the first 64 bytes of the initial page before doing the
first SHA256. Fix the loop to mix the store-clock-fast values
all over the page.

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>