ipvs: Update width of source for ip_vs_sync_conn_options

In ip_vs_sync_conn_v0() copy is made to struct ip_vs_sync_conn_options.
That structure looks like this:

struct ip_vs_sync_conn_options {
        struct ip_vs_seq        in_seq;
        struct ip_vs_seq        out_seq;
};

The source of the copy is the in_seq field of struct ip_vs_conn.  Whose
type is struct ip_vs_seq. Thus we can see that the source - is not as
wide as the amount of data copied, which is the width of struct
ip_vs_sync_conn_option.

The copy is safe because the next field in is another struct ip_vs_seq.
Make use of struct_group() to annotate this.

Flagged by gcc-13 as:

 In file included from ./include/linux/string.h:254,
                  from ./include/linux/bitmap.h:11,
                  from ./include/linux/cpumask.h:12,
                  from ./arch/x86/include/asm/paravirt.h:17,
                  from ./arch/x86/include/asm/cpuid.h:62,
                  from ./arch/x86/include/asm/processor.h:19,
                  from ./arch/x86/include/asm/timex.h:5,
                  from ./include/linux/timex.h:67,
                  from ./include/linux/time32.h:13,
                  from ./include/linux/time.h:60,
                  from ./include/linux/stat.h:19,
                  from ./include/linux/module.h:13,
                  from net/netfilter/ipvs/ip_vs_sync.c:38:
 In function 'fortify_memcpy_chk',
     inlined from 'ip_vs_sync_conn_v0' at net/netfilter/ipvs/ip_vs_sync.c:606:3:
 ./include/linux/fortify-string.h:529:25: error: call to '__read_overflow2_field' declared with attribute warning: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Werror=attribute-warning]
   529 |                         __read_overflow2_field(q_size_field, size);
       |

Compile tested only.
Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/ip_vs.h

@@ -630,8 +630,10 @@ struct ip_vs_conn {
 	 */
 	struct ip_vs_app        *app;           /* bound ip_vs_app object */
 	void                    *app_data;      /* Application private data */
-	struct ip_vs_seq        in_seq;         /* incoming seq. struct */
-	struct ip_vs_seq        out_seq;        /* outgoing seq. struct */
+	struct_group(sync_conn_opt,
+		struct ip_vs_seq  in_seq;       /* incoming seq. struct */
+		struct ip_vs_seq  out_seq;      /* outgoing seq. struct */
+	);
 
 	const struct ip_vs_pe	*pe;
 	char			*pe_data;

net/netfilter/ipvs/ip_vs_sync.c

@@ -603,7 +603,7 @@ static void ip_vs_sync_conn_v0(struct netns_ipvs *ipvs, struct ip_vs_conn *cp,
 	if (cp->flags & IP_VS_CONN_F_SEQ_MASK) {
 		struct ip_vs_sync_conn_options *opt =
 			(struct ip_vs_sync_conn_options *)&s[1];
-		memcpy(opt, &cp->in_seq, sizeof(*opt));
+		memcpy(opt, &cp->sync_conn_opt, sizeof(*opt));
 	}
 
 	m->nr_conns++;