LU-3683 libcfs: improve validity test for valid file descriptors

LU-3683 libcfs: improve validity test for valid file descriptors

File descriptor members of struct lustre_kernelcomm aren't set to
anything special at start time.
This leads to incorrect tests and skipping valid fd closes at stop time.
Fixed by defining, setting, and testing against a specific LK_NOFD value.

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I9306ce380c5e049ef442b99ac20e51271deafbc5
Reviewed-on: http://review.whamcloud.com/7275
Tested-by: Hudson
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>