LU-11894 lnet: check for asymmetrical route messages

LU-11894 lnet: check for asymmetrical route messages

Asymmetrical routes can be an issue when debugging network,
and allowing them also opens the door to attacks where hostile
clients inject data to the servers.

In order to prevent asymmetrical routes, add a new lnet kernel
module option named 'lnet_drop_asym_route'. When set to non-zero,
lnet_parse() will check if the message received from a remote peer
is coming through a router that would normally be used by this node
to reach the remote peer. If it is not the case, then it means we
are dealing with an asymmetrical route message, and the message will
be dropped.

The check for asymmetrical route can also be switched on/off with
the command 'lnetctl set drop_asym_route 0|1'. And this parameter is
exported/imported in Yaml.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I06fb23d9e46984d79c14fa9b53b2fa04ce3c50c5
Reviewed-on: https://review.whamcloud.com/34119
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>