s390/pai_crypto: Make pai_root per-PMU and unify naming

s390/pai_crypto: Make pai_root per-PMU and unify naming

Prepare the common PAI PMU driver to handle multiple PMUs.

Convert pai_root into an array indexed by PAI_PMU_IDX(event)
so that per-CPU state becomes per-PMU. Adjust all call sites
accordingly. Rename KMSG_COMPONENT and the s390dbf buffer from
"pai_crypto" to "pai" for consistent naming.

No functional change intended beyond log identifiers.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Jan Polensky <japo@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>