LU-17022 osd: convert od_connects to atomic_t

LU-17022 osd: convert od_connects to atomic_t

od_connects in ldiskfs is protected with od_osfs_lock.
in zfs it is protected with obd->obd_dev_lock.

If we convert it to atomic_t we get cheaper locking and less
confusion.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I4e28c22c8988c7f6a5e67064073541e917a209db
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51907
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>