namespace: don't log misleading error in the r > 0 path

fd_is_fs_type() returns < 0 for errors, 0 for false, and > 0 for true, so
in the r > branch we'd most likely report EPERM together with the error
message which is misleading.

src/core/namespace.c

@@ -3662,7 +3662,7 @@ static int unpeel_get_fd(const char *mount_path, int *ret_fd) {
                         _exit(EXIT_FAILURE);
                 }
                 if (r > 0) {
-                        log_debug_errno(r, "'%s' is still an overlay after opening mount tree: %m", mount_path);
+                        log_debug("'%s' is still an overlay after opening mount tree", mount_path);
                         _exit(EXIT_FAILURE);
                 }