LU-10391 lnet: change lp_disc_*_nid to struct lnet_nid

LU-10391 lnet: change lp_disc_*_nid to struct lnet_nid

Change lp_disc_src_nid and lp_disc_dst_nid in struct lnet_peer to
struct lnet_nid.

Test-Parameters: trivial
Test-Parameters: serverversion=2.12 serverdistro=el7.9 testlist=runtests
Test-Parameters: clientversion=2.12 testlist=runtests
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I0f127fbc790c0821900d7b8abfa56c1a7de8f944
Reviewed-on: https://review.whamcloud.com/44620
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>