MRP-1746 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 save to call ptlrpc_invalidate_import() when
import is deactive, but ptlrpc_reconnect_import() doesn't
deactivates it.
Let's use only pinger when available to recoonect import.

Change-Id: I24131a945b354b9e380bcf6b9dcb3510b2f39520
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://es-gerrit.xyus.xyratex.com:8080/1720
Reviewed-by: Rahul Deshmukh <rahul_deshmukh@xyratex.com>
Tested-by: Jenkins
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Tested-by: Andrey Bondarenko <andrey_bondarenko@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>