LU-12678 lnet: convert lpni_refcount to a kref

LU-12678 lnet: convert lpni_refcount to a kref

This refcount is used exactly like a kref.  So change it to one.
kref uses refcount_t which will warn on increment-from-zero and
similar problems (which enabled with CONFIG option), so we don't
need the LASSERT calls.

Change-Id: I857dff2c9838cb7d8f4b5f023f75f2d66119344f
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/41941
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>