LU-14627 lnet: Ensure ref taken when queueing for discovery

LU-14627 lnet: Ensure ref taken when queueing for discovery

Call lnet_peer_queue_for_discovery() in
lnet_discovery_event_handler() to ensure that we take a ref on
the peer when forcing it onto the discovery queue. This also ensures
that the peer state has LNET_PEER_DISCOVERING.

Add a test to sanity-lnet.sh that can trigger the refcount loss bug
in discovery.

HPE-bug-id: LUS-7651
Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Ie2908668c4ffde0f993b5b7ea9aa58acd1d6fa9c
Reviewed-on: https://review.whamcloud.com/43418
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Stephane Thiell <sthiell@stanford.edu>
Reviewed-by: Oleg Drokin <green@whamcloud.com>