fs: check access for unlinkat()

fs/vfs_ops.c

@@ -341,6 +341,12 @@ int vfs_unlinkat(struct vfs_ioctx *ctx, struct vnode *at, const char *pathname,
         }
     }
 
+    _assert(node->parent != NULL);
+    // Must have write access to remove entries
+    if ((res = vfs_access_node(ctx, node->parent, W_OK)) != 0) {
+        return res;
+    }
+
     if ((res = node->op->unlink(node)) != 0) {
         return res;
     }