LU-13892 lnet: lock-up during router check

LU-13892 lnet: lock-up during router check

This is a fix for the issue with LNet lock-up while waiting
for routers to become active with check_routers_before_use
option. Release ln_api_mutex while waiting to allow
incoming connections to be handled.

Signed-off-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Change-Id: I63b1d1ce5ee2b27a3bd2cea78713fc6fc7502cf7
Reviewed-on: https://review.whamcloud.com/40172
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>