ratelimit: Allow zero ->burst to disable ratelimiting

If ->interval is zero, then rate-limiting will be disabled.
Alternatively, if interval is greater than zero and ->burst is zero,
then rate-limiting will be applied unconditionally.  The point of this
distinction is to handle current users that pass zero-initialized
ratelimit_state structures to ___ratelimit(), and in such cases the
->lock field will be uninitialized.  Acquiring ->lock in this case is
clearly not a strategy to win.

Therefore, make this classification be lockless.

Note that although negative ->interval and ->burst happen to be treated
as if they were zero, this is an accident of the current implementation.
The semantics of negative values for these fields is subject to change
without notice.  Especially given that Bert Karwatzki determined that
no current calls to ___ratelimit() ever have negative values for these
fields.

This commit replaces an earlier buggy versions.

Link: https://lore.kernel.org/all/fbe93a52-365e-47fe-93a4-44a44547d601@paulmck-laptop/
Link: https://lore.kernel.org/all/20250423115409.3425-1-spasswolf@web.de/
Reported-by: Bert Karwatzki <spasswolf@web.de>
Reported-by: "Aithal, Srikanth" <sraithal@amd.com>
Closes: https://lore.kernel.org/all/20250423115409.3425-1-spasswolf@web.de/
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/257c3b91-e30f-48be-9788-d27a4445a416@sirena.org.uk/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: "Aithal, Srikanth" <sraithal@amd.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>

Author: paulmckrcu

lib/ratelimit.c

@@ -35,8 +35,12 @@ int ___ratelimit(struct ratelimit_state *rs, const char *func)
 	unsigned long flags;
 	int ret;
 
-	if (!interval)
-		return 1;
+	if (interval <= 0 || burst <= 0) {
+		ret = interval == 0 || burst > 0;
+		if (!ret)
+			ratelimit_state_inc_miss(rs);
+		return ret;
+	}
 
 	/*
 	 * If we contend on this state's lock then just check if