LU-16378 lnet: handles unregister/register events

LU-16378 lnet: handles unregister/register events

When network is restarted, devices are unregistered and then
registered again. When a device registers using an index that is
different from the previous one (before network was restarted), LNet
ignores it. Consequently, this device stays with link in fatal state.

To fix that, we catch unregistering events to clear the saved index
value, and when a registering event comes, we save the new value.

Signed-off-by: Cyril Bordage <cbordage@whamcloud.com>
Change-Id: I17e93a1103d588f3e630a9c7446b345f4d472b97