LU-13895 lnet: Prevent discovery on deleted peer

We needn't perform any discovery activities on a peer that has had
lnet_peer_del() called on it.

Test-Parameters: trivial
HPE-bug-id: LUS-9192
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I5c89dc89038d2c8bf4d2a29029af7720963b81a2
Reviewed-on: https://review.whamcloud.com/39605
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>