s390/zcrypt: Introduce cprb mempool for ep11 misc functions

s390/zcrypt: Introduce cprb mempool for ep11 misc functions

Introduce a cprb mempool for the zcrypt ep11 misc functions
(zcrypt_ep11misc.*) do some preparation rework to support
a do-not-allocate path through some zcrypt ep11 misc functions.

The mempool is controlled by the zcrypt module parameter
"mempool_threshold" which shall control the minimal amount
of memory items for CCA and EP11.

The mempool shall support "mempool_threshold" requests/replies
in parallel which means for EP11 to hold a send and receive
buffer memory per request. Each of this cprb space items is
limited to 8 KB. So by default the mempool consumes
  5 * 2 * 8KB = 80KB

If the mempool is depleted upon one ep11 misc functions is
called with the ZCRYPT_XFLAG_NOMEMALLOC xflag set, the function
will fail with -ENOMEM and the caller is responsible for taking
further actions.

This is only part of an rework to support a new xflag
ZCRYPT_XFLAG_NOMEMALLOC but not yet complete.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-8-freude@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>