LU-7734 lnet: set primary NID in ptlrpc_connection_get()

LU-7734 lnet: set primary NID in ptlrpc_connection_get()

Set the NID in ptlrpc_connection::c_peer to the primary NID of a peer.
This ensures that regardless of the NID used to start a connection, we
consistently use the same NID (the primary NID) to identify a peer. It
also means that PtlRPC will not create multiple connections to a peer.

The primary NID is obtained by calling LNetPrimaryNID(), an addition
to the exported symbols of the LNet module. The name was chosen to
match the existing naming pattern.

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: Idc0605d17a58678b634db246221028cf81ad2407
Reviewed-on: http://review.whamcloud.com/21710
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Tested-by: Amir Shehata <amir.shehata@intel.com>