LU-13605 lnet: Do not overwrite destination when routing

MR path selection in a routed environment is supposed to allow the
originator of a message to set the final destination NID. On a
multi-hop route, intermediate routers execute the same code path as
the message originator (i.e. the remote send cases). This causes
them to overwrite the destination NID when forwarding the message.

Check the msg_routing flag to determine whether we should set the
final destination NID (i.e. LNet peer NI).

A somewhat related issue is that because intermediate routers are not
selecting a destination lpni, they need to pick the next-hop lpni
based on the destination NID's remote net.

Test-Parameters: trivial
Fixes: 9dfdc2238be ("LU-13035 lnet: fix remote peer ni selection")
HPE-bug-id: LUS-8919
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Id2fbbc5d8da347e971bbb8ad2779e80f75e29dd7
Reviewed-on: https://review.whamcloud.com/38731
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>