LU-17022 obdclass: convert obd_conn_inprogress to atomic_t

LU-17022 obdclass: convert obd_conn_inprogress to atomic_t

Using atomic_t for obd_conn_inprogress means we don't need to take a
spinlock.
Also send wakeup when value reaches zero, and wait for the wakeup
instead of using a yield() loop.

Change-Id: I9af29e068203cde951e592c408906d121702fa18
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51906
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>