selftests: cpu-hotplug: fix case where CPUs offline > CPUs present

The cpu-hotplug test assumes that we can offline the maximum CPU as
described by /sys/devices/system/cpu/offline.  However, in the case
where the number of CPUs exceeds like kernel configuration then
the offline count can be greater than the present count and we end
up trying to test the offlining of a CPU that is not available to
offline.  Fix this by testing the maximum present CPU instead.

Also, the test currently offlines the CPU and does not online it,
so fix this by onlining the CPU after the test.

Fixes: d89dffa9 ("fault-injection: add selftests for cpu and memory hotplug")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>