LU-13713 lnet: check rtr_nid is a gateway

LU-13713 lnet: check rtr_nid is a gateway

The rtr_nid is specified for all REPLY/ACK. However it is possible
for the route through the gateway specified by rtr_nid to be removed.
In this case we don't want to use it. We should lookup alternative
paths.

This patch checks if the peer looked up is indeed a gateway. If it's
not a gateway then we attempt to find another path. There is no need
to fail right away. It's not a hard requirement to fail if the default
rtr_nid is not valid.

Test-Parameters: trivial
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: Ic1c93b7c6c2c8060e2cfeb8fb1cb875dbc3010f7
Reviewed-on: https://review.whamcloud.com/39175
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>