journald, ratelimit: fix inaccurate message suppression in journal_rate_limit_test() (#4291)

Currently, the ratelimit does not handle the number of suppressed messages accurately.
Even though the number of messages reaches the limit, it still allows to add one extra messages to journal.

This patch fixes the problem.

src/journal/journald-rate-limit.c

@@ -261,7 +261,7 @@ int journal_rate_limit_test(JournalRateLimit *r, const char *id, int priority, u
                 return 1 + s;
         }
 
-        if (p->num <= burst) {
+        if (p->num < burst) {
                 p->num++;
                 return 1;
         }