LU-1432 ptlrpc: LBUG in lprocfs_free_client_stats()

LU-1432 ptlrpc: LBUG in lprocfs_free_client_stats()

* serialize connect and target obd cleanup to avoid connect
  accessing unexisted data structure.
* connect export refcounting cleanup.

Signed-off-by: Hiroya Nozaki <nozaki.hiroya@jp.fujitsu.com>
Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Change-Id: I0a9e8a58ecdc1212565a478f4a758755a1b95f99
Reviewed-on: http://review.whamcloud.com/3244
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>