LU-14661 lnet: Check if discovery toggled off in ping reply

LU-14661 lnet: Check if discovery toggled off in ping reply

If a peer is initially discovered and found to have discovery
enabled, but the peer later reloads LNet with discovery disabled,
then we can delete the peer and re-create it the next time the peer
is discovered.

It is safe to delete and re-create the peer as long as it wasn't
configured manually.

In lnet_peer_deletion(), we need to use lnet_del_init() when removing
the peer from the discovery queue because the lnet_peer_del() code
path can result in a call to lnet_peer_queue_for_discovery() where
we check if the lp_dc_list is empty.

Lustre-change: https://review.whamcloud.com/43508
Lustre-commit: 143893381d428466d4c71e075a041a9cbbd28818

Test-Parameters: trivial
HPE-bug-id: LUS-9178
Fixes: aa7de0af69 ("LU-13895 lnet: Prevent discovery on peer marked deletion")
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I0b43d7541711a3b94c492082d4a29487ebe72b09
Reviewed-on: https://review.whamcloud.com/48296
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>