erspan: Initialize options_len before referencing options.

In the Linux kernel, the following vulnerability has been resolved: erspan: Initialize options_len before referencing options. The struct ip_tunnel_info has a flexible array member named options that is protected by a counted_by(options_len) attribute. The compiler will use this information to enforce runtime bounds checking deployed by FORTIFY_SOURCE string helpers. As laid out in the GCC documentation, the counter must be initialized before the first reference to the flexible array member. After scanning through the files that use struct ip_tunnel_info and also refer to options or options_len, it appears the normal case is to use the ip_tunnel_info_opts_set() helper. Said helper would initialize options_len properly before copying data into options, however in the GRE ERSPAN code a partial update is done, preventing the use of the helper function. Before this change the handling of ERSPAN traffic in GRE tunnels would cause a kernel panic when the kernel is compiled with GCC 15+ and having FORTIFY_SOURCE configured: memcpy: detected buffer overflow: 4 byte write of buffer size 0 Call Trace: <IRQ> __fortify_panic+0xd/0xf erspan_rcv.cold+0x68/0x83 ? ip_route_input_slow+0x816/0x9d0 gre_rcv+0x1b2/0x1c0 gre_rcv+0x8e/0x100 ? raw_v4_input+0x2a0/0x2b0 ip_protocol_deliver_rcu+0x1ea/0x210 ip_local_deliver_finish+0x86/0x110 ip_local_deliver+0x65/0x110 ? ip_rcv_finish_core+0xd6/0x360 ip_rcv+0x186/0x1a0 Reported-at: https://launchpad.net/bugs/2129580