exportfs: use dget_parent

Use dget_parent instead of opencoding it.  This simplifies the code, but
more importanly prepares for the more complicated locking for a parent
dget in the dcache scale patch series.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/exportfs/expfs.c

@@ -74,21 +74,20 @@ static struct dentry *
 find_disconnected_root(struct dentry *dentry)
 {
 	dget(dentry);
-	spin_lock(&dentry->d_lock);
-	while (!IS_ROOT(dentry) &&
-	       (dentry->d_parent->d_flags & DCACHE_DISCONNECTED)) {
-		struct dentry *parent = dentry->d_parent;
-		dget(parent);
-		spin_unlock(&dentry->d_lock);
+	while (!IS_ROOT(dentry)) {
+		struct dentry *parent = dget_parent(dentry);
+
+		if (!(parent->d_flags & DCACHE_DISCONNECTED)) {
+			dput(parent);
+			break;
+		}
+
 		dput(dentry);
 		dentry = parent;
-		spin_lock(&dentry->d_lock);
 	}
-	spin_unlock(&dentry->d_lock);
 	return dentry;
 }
 
-
 /*
  * Make sure target_dir is fully connected to the dentry tree.
  *