bpf: Use struct_size()

Use struct_size() instead of hand writing it.
This is less verbose and more robust.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240429121323.3818497-1-andriy.shevchenko@linux.intel.com

kernel/bpf/core.c

@@ -26,6 +26,7 @@
 #include <linux/bpf.h>
 #include <linux/btf.h>
 #include <linux/objtool.h>
+#include <linux/overflow.h>
 #include <linux/rbtree_latch.h>
 #include <linux/kallsyms.h>
 #include <linux/rcupdate.h>
@@ -2455,13 +2456,14 @@ EXPORT_SYMBOL(bpf_empty_prog_array);
 
 struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags)
 {
+	struct bpf_prog_array *p;
+
 	if (prog_cnt)
-		return kzalloc(sizeof(struct bpf_prog_array) +
-			       sizeof(struct bpf_prog_array_item) *
-			       (prog_cnt + 1),
-			       flags);
+		p = kzalloc(struct_size(p, items, prog_cnt + 1), flags);
+	else
+		p = &bpf_empty_prog_array.hdr;
 
-	return &bpf_empty_prog_array.hdr;
+	return p;
 }
 
 void bpf_prog_array_free(struct bpf_prog_array *progs)