[S390] smp: Merge smp_count_cpus() and smp_get_save_areas().

Merge smp_count_cpus() and smp_get_save_areas() so we save a loop over
all potentially present cpus.

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