s390/zcrypt: Rework ep11 misc functions to use cprb mempool

There are two places in the ep11 misc code where a short term
memory buffer is needed. Rework this code to use the cprb mempool
to satisfy this ephemeral memory requirements.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-19-freude@linux.ibm.com


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