LU-12756 lnet: Remove unnecessary rtr_nid argument

LU-12756 lnet: Remove unnecessary rtr_nid argument

Cache the rtr_nid argument in lnet_select_pathway() the same way we
cache the src_nid argument.

Also remove the unnecessary lnet_nid_t variable that stores the
lp_primary_nid solely for the purposes of printing a debug message.

Test-Parameters: trivial
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I0a265bbb1c57eba0373a38fbacacceb64faf4614
Reviewed-on: https://review.whamcloud.com/36540
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>