LU-4913 mgc: mgc import reconnect race

LU-4913 mgc: mgc import reconnect race

mgc import can be reconnected by pinger or
ptlrpc_reconnect_import().
ptlrpc_invalidate_import() isn't protected against
alteration of imp_invalid state. Import can be
reconnected by pinger which makes imp_invalid
equal to false. Thus LASSERT(imp->imp_invalid) fails
in ptlrpc_invalidate_import().

It is safe to call ptlrpc_invalidate_import() when
import is deactivated, but ptlrpc_reconnect_import() doesn't
deactivate it.
Let's use only pinger when available to reconnect import

Xyratex-bug-id: MRP-1746
Change-Id: I2feb45c5f3e96da30dd5639d5824068f8a126c7d
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://review.whamcloud.com/9967
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>