LU-13277 lnet: Discovery thread can deadlock on shutdown

LU-13277 lnet: Discovery thread can deadlock on shutdown

Drop the net_lock/EX before breaking out of the loop to avoid
deadlock.

Cray-bug-id: LUS-8525
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: Ie62b55fa45b5795937eb1480a1fcabe295fed0ee
Reviewed-on: https://review.whamcloud.com/37675
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>