LU-13895 lnet: Prevent discovery on deleted peer

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.

Lustre-change: https://review.whamcloud.com/39605
Lustre-commit: fd32cd817cba336c684fe3ab7aac79705061e8b5

Test-Parameters: trivial testlist=sanity-lnet
HPE-bug-id: LUS-9192
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I5c89dc89038d2c8bf4d2a29029af7720963b81a2
Reviewed-on: https://review.whamcloud.com/48292
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>