watchdog/perf: one digit too short in the raw event config copy

Commit 6164be01f1 ("watchdog/perf: optimize bytes copied and remove
manual NUL-termination") swapped strscpy(buf, str, sizeof(buf)) plus a
manual buf[len] = 0 for strscpy(buf, str, len), and that count is one
short.  strscpy() keeps the last byte of the destination for the NUL, so
the config loses its final digit, nmi_watchdog=r300,panic for example ends
up with buf = "30" and arms the raw event with the wrong config.

The empty case falls over too, strscpy() with a zero count writes nothing
at all, so nmi_watchdog=r,1 leaves buf uninitialized and kstrtoull() reads
whatever stack garbage is sitting there.

The old code was safe on both by accident, it filled the whole buffer
first and buf[len] = 0 then stomped the comma position, so the worst you
got was a truncated parse failure.

Pass len + 1 so the copy includes the character the NUL replaces, and
reject len >= sizeof(buf) like it was before the optimization, which also
turns an empty config back into a clean parse failure.

Link: https://lore.kernel.org/20261003110200.3-brads@mainlining.org
Fixes: 6164be01f1 ("watchdog/perf: optimize bytes copied and remove manual NUL-termination")
Signed-off-by: Bradley Morgan <brads@mainlining.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: Thorsten Blum <blum@kernel.org>
Cc: Thomas Gleixner <tglx@kernel.org>

kernel/watchdog_perf.c

@@ -301,10 +301,10 @@ void __init hardlockup_config_perf_event(const char *str)
 	} else {
 		unsigned int len = comma - str;
 
-		if (len > sizeof(buf))
+		if (len >= sizeof(buf))
 			return;
 
-		strscpy(buf, str, len);
+		strscpy(buf, str, len + 1);
 		if (kstrtoull(buf, 16, &config))
 			return;
 	}