samples/bpf: fix a compilation failure

samples/bpf build failed with the following errors:

  $ make samples/bpf/
  ...
  HOSTCC  samples/bpf/sockex3_user.o
  /data/users/yhs/work/net-next/samples/bpf/sockex3_user.c:16:8: error: redefinition of ‘struct bpf_flow_keys’
   struct bpf_flow_keys {
          ^
  In file included from /data/users/yhs/work/net-next/samples/bpf/sockex3_user.c:4:0:
  ./usr/include/linux/bpf.h:2338:9: note: originally defined here
    struct bpf_flow_keys *flow_keys;
           ^
  make[3]: *** [samples/bpf/sockex3_user.o] Error 1

Commit d58e468b ("flow_dissector: implements flow dissector BPF hook")
introduced struct bpf_flow_keys in include/uapi/linux/bpf.h and hence
caused the naming conflict with samples/bpf/sockex3_user.c.

The fix is to rename struct bpf_flow_keys in samples/bpf/sockex3_user.c
to flow_keys to avoid the conflict.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>