LU-12756 lnet: Refactor lnet_find_existing_preferred_best_ni

Replace lnet_send_data argument.

Get rid of unnecessary lookup for lnet_peer_net.

Test-Parameters: trivial
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I69e733d4a0af55ec480df4a13e9153757212333e
Reviewed-on: https://review.whamcloud.com/36538
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexandr Boyko <c17825@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>