s390/crypto: simplify CPACF encryption / decryption functions

The double while loops of the CTR mode encryption / decryption functions
are overly complex for little gain. Simplify the functions to a single
while loop at the cost of an additional memcpy of a few bytes for every
4K page worth of data.
Adapt the other crypto functions to make them all look alike.

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