LU-13472 lnet: set route aliveness properly

LU-13472 lnet: set route aliveness properly

In the case when the discover is toggled from on to off, the route
aliveness might become stale due to not updating the route->lr_alive
variable correctly. It will get updated once the gateway is pinged.
However, there is a period of max alive_router_check_interval where
the route can be down.

Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: Ic1754d6e7ddc9398efc7a64f823a70e5546e9ca6
Reviewed-on: https://review.whamcloud.com/38323
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>