extmod/modlwip: Fix IGMP address type when IPv6 is enabled.

This was missed in 628abf8f25.  The the bug
was that, when IPv6 is enabled, the `sizeof(ip_addr_t)` is much larger than
IPv4 size, which is what's needed for IGMP addressing.

Fixes issue #16100.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modlwip.c

@@ -1432,7 +1432,7 @@ static mp_obj_t lwip_socket_setsockopt(size_t n_args, const mp_obj_t *args) {
         case IP_DROP_MEMBERSHIP: {
             mp_buffer_info_t bufinfo;
             mp_get_buffer_raise(args[3], &bufinfo, MP_BUFFER_READ);
-            if (bufinfo.len != sizeof(ip_addr_t) * 2) {
+            if (bufinfo.len != sizeof(MP_IGMP_IP_ADDR_TYPE) * 2) {
                 mp_raise_ValueError(NULL);
             }