LU-17207 lnet: race b/w monitor thr stop and discovery push

LU-17207 lnet: race b/w monitor thr stop and discovery push

As a result of race, discovery thread may attempt to dereference
a message on ln_mt_resendqs which was just freed by monitor thread
stopping. Make sure discovery thread is stopped first.

Lustre-change: https://review.whamcloud.com/52734/
Lustre-commit: TBD (from 5c6ca4991382a805da6e824c1dbfab931987dda6)

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I0dfcf3bc5bb3c8df195388599f571bdd3caaa3d7
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52935
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>