LU-9480 lnet: refactor lnet_add_peer_ni()

LU-9480 lnet: refactor lnet_add_peer_ni()

Refactor lnet_add_peer_ni() and the functions called by it. In
particular, lnet_peer_add_nid() adds an lnet_peer_ni to an
existing lnet_peer, lnet_peer_add() adds a new lnet_peer.

lnet_find_or_create_peer_locked() is removed.

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: Iffcbf9ffc26460afc544f102bd0e0a56e23a83f3
Reviewed-on: https://review.whamcloud.com/25780
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Tested-by: Amir Shehata <amir.shehata@intel.com>