net: ip_tunnel: initialize `options_len` before referencing options

In the Linux kernel, the following vulnerability has been resolved:

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.