s390/prng: Adjust generation of entropy to produce real 256 bits.

The generate_entropy function used a sha256 for compacting
together 256 bits of entropy into 32 bytes hash. However, it
is questionable if a sha256 can really be used here, as
potential collisions may reduce the max entropy fitting into
a 32 byte hash value. So this batch introduces the use of
sha512 instead and the required buffer adjustments for the
calling functions.

Further more the working buffer for the generate_entropy
function has been widened from one page to two pages. So now
1024 stckf invocations are used to gather 256 bits of
entropy. This has been done to be on the save side if the
jitters of stckf values isn't as good as supposed.

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