manager: only issue overmount warning when the check succeeded

If for any reason the check failed (selinux?), we would still issue
the warning. Check the return status.

(cherry picked from commit 056edeb910)

src/core/mount.c

@@ -813,19 +813,26 @@ fail:
 }
 
 void warn_if_dir_nonempty(const char *unit, const char* where) {
+        int r;
+
         assert(unit);
         assert(where);
 
-        if (dir_is_empty(where) > 0)
+        r = dir_is_empty(where);
+        if (r > 0)
                 return;
-
-        log_struct_unit(LOG_NOTICE,
-                   unit,
-                   "MESSAGE=%s: Directory %s to mount over is not empty, mounting anyway.",
-                   unit, where,
-                   "WHERE=%s", where,
-                   MESSAGE_ID(SD_MESSAGE_OVERMOUNTING),
-                   NULL);
+        else if (r == 0)
+                log_struct_unit(LOG_NOTICE,
+                                unit,
+                                "MESSAGE=%s: Directory %s to mount over is not empty, mounting anyway.",
+                                unit, where,
+                                "WHERE=%s", where,
+                                MESSAGE_ID(SD_MESSAGE_OVERMOUNTING),
+                                NULL);
+        else
+                log_warning_unit(unit,
+                                 "MESSAGE=Failed to check directory %s: %s",
+                                 where, strerror(-r));
 }
 
 static int fail_if_symlink(const char *unit, const char* where) {