net: ip_tunnel: initialize `options_len` before referencing options

The following command triggers a kernel panic:

  ip link add d0 type dummy; ip link set d0 up
  ip route add 10.30.0.0/16 \
    encap ip id 300 geneve_opts 4660:66:11223344 dev d0

  memcpy: detected buffer overflow: 4 byte write of buffer size 0
  kernel BUG at lib/string_helpers.c:1044!
  ...
  ip_tun_parse_opts.part.0.cold+0x10/0x10
  ip_tun_build_state+0x116/0x2a0

On kernels built with GCC 15+ and `CONFIG_FORTIFY_SOURCE`, the fortified
`memcpy()` got 0 sized destination with request of 4 bytes length:

  static int ip_tun_parse_opts_geneve(...)
  {
      ...
      attr = tb[LWTUNNEL_IP_OPT_GENEVE_DATA];
      data_len = nla_len(attr); /* == 4 */

      struct geneve_opt *opt = ip_tunnel_info_opts(info) + opts_len;
      memcpy(opt->opt_data, nla_data(attr), data_len);
      /*     ^^^^^^^^^^^^^ 0 since options_len is assigned afterwards */

Fixed by initializing the counter before the options are referenced.
Matching what `tunnel_key_opts_set()` already does.

Fixes: bb5e62f2d547 ("net: Add options as a flexible array to struct ip_tunnel_info")
Cc: stable@vger.kernel.org
Signed-off-by: Gris Ge <cnfourt@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/20260913090851.468216-1-cnfourt@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>