s390/zcrypt: Rework cca misc functions kmallocs to use the cprb mempool

Rework two places in the zcrypt cca misc code using kmalloc() for
ephemeral memory allocation. As there is anyway now a cprb mempool
let's use this pool instead to satisfy these short term memory
allocations.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-16-freude@linux.ibm.com


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