LU-14048 obd: fix race between connect vs disconnect

LU-14048 obd: fix race between connect vs disconnect

The export nid hash would be removed in class_disconnect, But
still a race window exists in target_handle_connect to add it back.
Then the process of cleanup will wait infinity.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I9ad3edbd040b81e2aef7ae22494302d9a478d65b
Reviewed-on: https://review.whamcloud.com/41687
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>