bpf prog: offload.c - add stub sched_cls_ingress_tether_rawip for 4.9 kernels

This allows this program to be correctly loaded and attached on 4.9 devices,
thus reducing higher level stack complexity.

Example on aosp cuttlefish 5.4.30 virtual device:
  D LibBpfLoader: bpf_prog_load lib call for /system/etc/bpf/offload.o (schedcls_ingress_tether_ether) returned fd: 8 (no error)
  D LibBpfLoader: cs[1].name:schedcls_ingress_tether_rawip$stub min_kver:0 .max_kver:40e00 (kvers:5041e)
  D LibBpfLoader: cs[2].name:schedcls_ingress_tether_rawip$4_14 min_kver:40e00 .max_kver:ffffffff (kvers:5041e)
  D LibBpfLoader: bpf_prog_load lib call for /system/etc/bpf/offload.o (schedcls_ingress_tether_rawip$4_14) returned fd: 9 (no error)
  I bpfloader: Attempted load object: /system/etc/bpf/offload.o, ret: Success

  $ adb shell ls -l /sys/fs/bpf | egrep offload
  -rw-rw---- 1 root network_stack 0 2020-04-22 01:27 map_offload_tether_ingress_map
  -rw-rw---- 1 root network_stack 0 2020-04-22 01:27 map_offload_tether_stats_map
  -r--r----- 1 root root          0 2020-04-22 01:27 prog_offload_schedcls_ingress_tether_ether
  -r--r----- 1 root root          0 2020-04-22 01:27 prog_offload_schedcls_ingress_tether_rawip

Test: builds, atest, proper program loaded on 5.4.30 aosp cuttlefish
Bug: 153437886
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I150bdb2d1da5b79189156ed09ffd5f063ed90709
Merged-In: I150bdb2d1da5b79189156ed09ffd5f063ed90709