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.

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I0dfcf3bc5bb3c8df195388599f571bdd3caaa3d7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52734
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>