Helpers like ip_tunnel_info_opts_{get,set}() are only available if
CONFIG_INET is set, thus add an empty definition into the header for
the !CONFIG_INET case, where already other empty inline helpers are
defined.

This avoids ifdef kludge inside filter.c, but also vxlan and geneve
themself where this facility can only be used with, depend on INET
being set. For the !INET case TUNNEL_OPTIONS_PRESENT would never be
set in flags.

Fixes: 14ca0751 ("bpf: support for access to tunnel options")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>