clk: renesas: cpg-mssr: Use struct_size() helper

struct_size() is what is normally used when a flexible array member is
present to avoid accidental mistakes.  pm_size is still needed for the
memcpy() call below.

Added __counted_by for extra runtime analysis.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260324021103.13651-1-rosenp@gmail.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Author: neheb

drivers/clk/renesas/renesas-cpg-mssr.c

@@ -569,7 +569,7 @@ static void __init cpg_mssr_register_mod_clk(const struct mssr_mod_clk *mod,
 struct cpg_mssr_clk_domain {
 	struct generic_pm_domain genpd;
 	unsigned int num_core_pm_clks;
-	unsigned int core_pm_clks[];
+	unsigned int core_pm_clks[] __counted_by(num_core_pm_clks);
 };
 
 static struct cpg_mssr_clk_domain *cpg_mssr_clk_domain;
@@ -667,7 +667,7 @@ static int __init cpg_mssr_add_clk_domain(struct device *dev,
 	size_t pm_size = num_core_pm_clks * sizeof(core_pm_clks[0]);
 	int ret;
 
-	pd = devm_kzalloc(dev, sizeof(*pd) + pm_size, GFP_KERNEL);
+	pd = devm_kzalloc(dev, struct_size(pd, core_pm_clks, num_core_pm_clks), GFP_KERNEL);
 	if (!pd)
 		return -ENOMEM;