LU-14283 obdclass: connect vs disconnect race

LU-14283 obdclass: connect vs disconnect race

There might be a possible race if setup (connect)
and cleanup (disconnect) are tangled together(similar
comments in osc_disconnect()):

  Thread1:				Thread2:
   connecting				 class_cleanup
     ptlrpc_connect_interpret
					   obd->obd_setup = 0
	obd_import_event
	  if (obd->obd_set_up)
		osc_init_grant() /*skipped*/
        ptlrpc_activate_import..

And If RPC was waked up and send out before
class_disconnect_exports(), It might hit divide zero crash
in osc_announce_cached() because @cl_max_extent_pages is zero.

The problem is we clear @obd_setup too early, It should be cleared
when OBD is really shutdown.

Fixes: 45900a ("LU-4134 obdclass: obd_device improvement")
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I898b6f53602c05221a3154a61615a0e270167ac6
Reviewed-on: https://review.whamcloud.com/41256
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>