LU-4454 libcfs: warn if all HTs in a core are gone

libcfs cpu partition can't support CPU hotplug, but it is safe
when plug-in new CPU or enabling/disabling hyper-threading.
It has potential risk only if plug-out CPU because it may break CPU
affinity of Lustre threads.

Current libcfs will print warning for all CPU notification, this
patch changed this behavior and only output warning when we lost all
HTs in a CPU core which may have broken affinity of Lustre threads.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Change-Id: I62267b62871c129beeb1593c4f69e7b81a79999d
Reviewed-on: http://review.whamcloud.com/8770
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>