kernel/core_hook.c: use warning instead

Tell builders that they can backport path_umount and point them to KernelSU's documentation.

Author: backslashxx

kernel/core_hook.c

@@ -519,7 +519,9 @@ static void ksu_umount_mnt(struct path *path, int flags)
 		pr_info("umount %s failed: %d\n", path->dentry->d_iname, err);
 	}
 #else
-  #error Make sure to backport path_umount to fs/namespace.c from 5.9! Read: https://kernelsu.org/guide/how-to-integrate-for-non-gki.html#backport-path_umount-from-linux-59
+	// TODO: umount for non GKI kernel
+	#warning Did you know you can backport path_umount to fs/namespace.c from 5.9?
+	#warning https://kernelsu.org/guide/how-to-integrate-for-non-gki.html#backport-path_umount-from-linux-59
 #endif
 }