net: core: annotate socks of struct sock_reuseport with __counted_by

According to '__reuseport_alloc()', annotate flexible array member
'sock' of 'struct sock_reuseport' with '__counted_by()' and use
convenient 'struct_size()' to simplify the math used in 'kzalloc()'.
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240801142311.42837-1-dmantipov@yandex.ruSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sock_reuseport.h

@@ -26,7 +26,7 @@ struct sock_reuseport {
 	unsigned int		bind_inany:1;
 	unsigned int		has_conns:1;
 	struct bpf_prog __rcu	*prog;		/* optional BPF sock selector */
-	struct sock		*socks[];	/* array of sock pointers */
+	struct sock		*socks[] __counted_by(max_socks);
 };
 
 extern int reuseport_alloc(struct sock *sk, bool bind_inany);

net/core/sock_reuseport.c

@@ -173,10 +173,9 @@ static bool __reuseport_detach_closed_sock(struct sock *sk,
 
 static struct sock_reuseport *__reuseport_alloc(unsigned int max_socks)
 {
-	unsigned int size = sizeof(struct sock_reuseport) +
-		      sizeof(struct sock *) * max_socks;
-	struct sock_reuseport *reuse = kzalloc(size, GFP_ATOMIC);
+	struct sock_reuseport *reuse;
 
+	reuse = kzalloc(struct_size(reuse, socks, max_socks), GFP_ATOMIC);
 	if (!reuse)
 		return NULL;