selftests/bpf: Fix up xdp bonding test wrt feature flags

Adjust the XDP feature flags for the bond device when no bond slave
devices are attached. After 9b0ed890 ("bonding: do not report
NETDEV_XDP_ACT_XSK_ZEROCOPY"), the empty bond device must report 0
as flags instead of NETDEV_XDP_ACT_MASK.

  # ./vmtest.sh -- ./test_progs -t xdp_bond
  [...]
  [    3.983311] bond1 (unregistering): (slave veth1_1): Releasing backup interface
  [    3.995434] bond1 (unregistering): Released all slaves
  [    4.022311] bond2: (slave veth2_1): Releasing backup interface
  #507/1   xdp_bonding/xdp_bonding_attach:OK
  #507/2   xdp_bonding/xdp_bonding_nested:OK
  #507/3   xdp_bonding/xdp_bonding_features:OK
  #507/4   xdp_bonding/xdp_bonding_roundrobin:OK
  #507/5   xdp_bonding/xdp_bonding_activebackup:OK
  #507/6   xdp_bonding/xdp_bonding_xor_layer2:OK
  #507/7   xdp_bonding/xdp_bonding_xor_layer23:OK
  #507/8   xdp_bonding/xdp_bonding_xor_layer34:OK
  #507/9   xdp_bonding/xdp_bonding_redirect_multi:OK
  #507     xdp_bonding:OK
  Summary: 1/9 PASSED, 0 SKIPPED, 0 FAILED
  [    4.185255] bond2 (unregistering): Released all slaves
  [...]

Fixes: 9b0ed890

 ("bonding: do not report NETDEV_XDP_ACT_XSK_ZEROCOPY")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Message-ID: <20240305090829.17131-2-daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>