s390/pai_crypto: Unify sample push logic and update context handling

s390/pai_crypto: Unify sample push logic and update context handling

Unify naming and logic for PAI PMU drivers to support both PMUs
pai_crypto and pai_ext.

Rename paicrypt_push_sample() to pai_push_sample() to reflect
its common usage.  Add detailed comments about invocation context
and scheduler callbacks.  Use struct pai_pmu to determine area_size
instead of PAGE_SIZE for counter backup.
Remove obsolete variable paicrypt_cnt.

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>