s390/smp: limit number of cpus in possible cpu mask

Limit the number of bits to the maximum number of cpus a machine
can have.
possible_cpu_mask typically will have more bits set than a machine
may physically have. This results in wasted memory during per-cpu
memory allocations, if the possible mask contains more cpus than
physically possible for a given configuration.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>