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

Lustre-change: https://review.whamcloud.com/39933
Lustre-commit: e0c445648a38fb72cc426ac0c16c33f5183cda08

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: If0886f1bc4412dd9cacb08a0f06fa69aeeed1c5b
Reviewed-on: https://review.whamcloud.com/40171
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>