__detach_mounts(): lookup_mountpoint() can't return ERR_PTR() anymore

... not since 1e9c75fb ("mnt: fix __detach_mounts infinite loop")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1625,7 +1625,7 @@ void __detach_mounts(struct dentry *dentry)
 	namespace_lock();
 	lock_mount_hash();
 	mp = lookup_mountpoint(dentry);
-	if (IS_ERR_OR_NULL(mp))
+	if (!mp)
 		goto out_unlock;
 
 	event++;