LU-10391 lnet: extend prefered nids in struct lnet_peer_ni

union lpni_pref in struct lnet_peer_ni how has 'struct lnet_nid'
rather than lnet_nid_t.

Also, lnet_peer_ni_set_no_mr_pref_nid() allows the pref nid to be NULL
and is a no-op in that case.

Rather then updating the user-space cfs_match_nid_net() in
libcfs/utils/nidstrings.c, remove it as it is unused.

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: I9a2453185aa5d708e6939dadc1e954c9dbd24efc
Reviewed-on: https://review.whamcloud.com/43596
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: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>