s390/zcrypt: Introduce cprb mempool for cca misc functions

s390/zcrypt: Introduce cprb mempool for cca misc functions

Introduce a new module parameter "zcrypt_mempool_threshold"
for the zcrypt module. This parameter controls the minimal
amount of mempool items which are pre-allocated for urgent
requests/replies and will be used with the support for the
new xflag ZCRYPT_XFLAG_NOMEMALLOC. The default value of 5
shall provide enough memory items to support up to 5 requests
(and their associated reply) in parallel. The minimum value
is 1 and is checked in zcrypt module init().

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

For CCA each mempool item is 16KB, as a CCA CPRB needs to
hold the request and the reply. The pool items only support
requests/replies with a limit of about 8KB.
So by default the CCA mempool consumes
  5 * 16KB = 80KB

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/20250424133619.16495-7-freude@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>