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If we're sending a discovery PUSH after receiving a discovery
REPLY then we want to send via the same NID that the reply was
sent to. This introduces a challenge in selecting an appropriate
destination NID for the PUSH because lnet_select_pathway() will not
run the MR selection algorithm for choosing a peer NI if the source
NI has been specified.
It is reasonable to assume that the NID used by the message
originator in sending the REPLY is a suitable destination for the
discovery PUSH. Thus, we record this NID in the same location we
currently record the lp_disc_src_nid, and use it when sending the
PUSH. With this change, the only other user of lnet_peer_select_nid()
is lnet_peer_send_ping(). In the ping case we do not set a source NID,
so lnet_select_pathway() is free to choose any peer NI. So this change
allows us to get rid of lnet_peer_select_nid() altogether.
Alternatively, we would need to reproduce a lot of the path selection
algorithm inside lnet_peer_select_nid() in order to avoid sending to
unhealthy NIDs. It seems undesirable and unnecessary to duplicate that
logic.
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