s390/appldata_os: fix cpu array size calculation

The cpu array size calculation uses the NR_CPUS config option, which
was recently increased from 64 to 256. With a value of 256, the cpu
array will no longer fit into one APPLDATA record and loading the
appldata_os module fails with the following error:
could not insert 'appldata_os': Cannot allocate memory

Use num_possible_cpus() instead of NR_CPUS. For z/VM, this will still
result in a value of 64. This is not true for LPAR, but the appldata
feature is not available for LPAR.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>