dlm: fix plock use-after-free

Fix a regression from the original addition of nfs lock support
586759f0.  When a synchronous
(non-nfs) plock completes, the waiting thread will wake up and
free the op struct.  This races with the user thread in
dev_write() which goes on to read the op's callback field to
check if the lock is async and needs a callback.  This check
can happen on the freed op.  The fix is to note the callback
value before the op can be freed.
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/plock.c

@@ -353,7 +353,7 @@ static ssize_t dev_write(struct file *file, const char __user *u, size_t count,
 {
 	struct dlm_plock_info info;
 	struct plock_op *op;
-	int found = 0;
+	int found = 0, do_callback = 0;
 
 	if (count != sizeof(info))
 		return -EINVAL;
@@ -366,21 +366,24 @@ static ssize_t dev_write(struct file *file, const char __user *u, size_t count,
 
 	spin_lock(&ops_lock);
 	list_for_each_entry(op, &recv_list, list) {
-		if (op->info.fsid == info.fsid && op->info.number == info.number &&
+		if (op->info.fsid == info.fsid &&
+		    op->info.number == info.number &&
 		    op->info.owner == info.owner) {
+			struct plock_xop *xop = (struct plock_xop *)op;
 			list_del_init(&op->list);
-			found = 1;
-			op->done = 1;
 			memcpy(&op->info, &info, sizeof(info));
+			if (xop->callback)
+				do_callback = 1;
+			else
+				op->done = 1;
+			found = 1;
 			break;
 		}
 	}
 	spin_unlock(&ops_lock);
 
 	if (found) {
-		struct plock_xop *xop;
-		xop = (struct plock_xop *)op;
-		if (xop->callback)
+		if (do_callback)
 			dlm_plock_callback(op);
 		else
 			wake_up(&recv_wq);