LU-12893 lnet: fix peer_ni selection

LU-12893 lnet: fix peer_ni selection

When selecting a peer-ni we must use the same peer NID
through all the messages which belong to the same RPC.
This is necessary in order to ensure we do the RDMA over
the optimal interface.

Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I0391537da32bc6ac7a8a3d92e207bf172d111981
Reviewed-on: https://review.whamcloud.com/36552
Reviewed-by: Chris Horn <hornc@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>