LU-12222 lnet: Introduce constant for the lolnd NID

LU-12222 lnet: Introduce constant for the lolnd NID

This patch adds a new constant, LNET_NID_LO_0, to represent the lolnd
NID 0@lo.

Lustre-change: https://review.whamcloud.com/38312
Lustre-commit: 56203e4ba0a64789e42ea45946e8c51f1db351fb

HPE-bug-id: LUS-8457
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I3e57637f297b8de306905a447af8f025e31d1fcf
Reviewed-on: https://review.whamcloud.com/38863
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>