LU-12616 obclass: fix MDS start/stop race

LU-12616 obclass: fix MDS start/stop race

The MDS unload happen when type of MDT has no reference.
The MDT drop it during obd_cleanup. So race window located
between obd_cleanup and server_stop_servers.
Lustre can lost MDS obd_device during server_start_targets
between MDS checking and taking the type reference, if another MDT
stops.

The patch takes one more reference for a MDT type at
server_start_targets, and put it at server_stop_servers.

This patch adds sanity test 278. It reproduces the next race
   started cleanup of MDT01
   started cleanup of MDT00
   finished cleanup of MDT00
   started MDT00 mount, checked MDS exist
   finished cleanup of MDT01, and cleanup of MDS also
   asserted during MDT00 initialization

Cray-bug-id: LUS-7275
Signed-off-by: Alexander Boyko <c17825@cray.com>
Change-Id: I9ae3bc2ec1d23c8d436f143d12e26209fdb6b083
Reviewed-on: https://review.whamcloud.com/35652
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>