EDAC: Fix workqueue-related crashes

00740c58 changed edac_core to
un-/register a workqueue item only if a lowlevel driver supplies a
polling routine. Normally, when we remove a polling low-level driver, we
go and cancel all the queued work. However, the workqueue unreg happens
based on the ->op_state setting, and edac_mc_del_mc() sets this to
OP_OFFLINE _before_ we cancel the work item, leading to NULL ptr oops on
the workqueue list.

Fix it by putting the unreg stuff in proper order.

Cc: <stable@kernel.org> #36.x
Reported-and-tested-by: Tobias Karnat <tobias.karnat@googlemail.com>
LKML-Reference: <1291201307.3029.21.camel@Tobias-Karnat>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>

drivers/edac/edac_mc.c

@@ -586,14 +586,16 @@ struct mem_ctl_info *edac_mc_del_mc(struct device *dev)
 		return NULL;
 	}
 
-	/* marking MCI offline */
-	mci->op_state = OP_OFFLINE;
-
 	del_mc_from_global_list(mci);
 	mutex_unlock(&mem_ctls_mutex);
 
-	/* flush workq processes and remove sysfs */
+	/* flush workq processes */
 	edac_mc_workq_teardown(mci);
+
+	/* marking MCI offline */
+	mci->op_state = OP_OFFLINE;
+
+	/* remove from sysfs */
 	edac_remove_sysfs_mci_device(mci);
 
 	edac_printk(KERN_INFO, EDAC_MC,