select/linear: Correct CPU count

Correct count of CPUs allocated to job on system with hyperthreads.
The bug was introduced in commit a6d3074d
On a system with hyperthreads:
srun -n1 --ntasks-per-core=1 hostname
you would get:
slurmctld: error: job_update_cpu_cnt: cpu_cnt underflow on job_id 67072