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>