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