geneve: Avoid -Wflex-array-member-not-at-end warning

geneve: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the corresponding
structure. Notice that `struct ip_tunnel_info` is a flexible
structure, this is a structure that contains a flexible-array
member.

Fix the following warning:

drivers/net/geneve.c:56:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/aMBK78xT2fUnpwE5@kspp
Signed-off-by: Jakub Kicinski <kuba@kernel.org>