LU-12233 lnet: deadlock on LNet shutdown

LU-12233 lnet: deadlock on LNet shutdown

Release ln_api_mutex during LNet shutdown while waiting
for zombie LNI to allow other threads to read the LNet
state updated by the shutdown and fall through, avoiding
the deadlock

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: Iaba11624d5b79bd0acb4add39f6153c55770440a
Reviewed-on: https://review.whamcloud.com/39933
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>