cpupower: Fix checks for CPU existence

cpupower: Fix checks for CPU existence

Calls to cpufreq_cpu_exists(cpu) were converted to
cpupower_is_cpu_online(cpu) when libcpupower was introduced and the
former function was deleted.  However, cpupower_is_cpu_online() does
not distinguish physically absent and offline CPUs, and does not set
errno.

cpufreq-set has already been fixed (commit c25badc9ceb6).

In cpufreq-bench, which prints an error message for offline CPUs,
properly distinguish and report the zero and negative cases.

Fixes: ac5a181d065d ("cpupower: Add cpuidle parts into library")
Fixes: 53d1cd6b125f ("cpupowerutils: bench - Fix cpu online check")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[carnil: Update/Refresh patch for 4.14.17: The issue with the
incorrect check has been fixed with upstream commit 53d1cd6b125f.
Keep in the patch the distinction and report for the zero and
negative cases.]

Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name cpupower-fix-checks-for-cpu-existence.patch