selftests/bpf: add test for xdp_master_redirect with bond not up

selftests/bpf: add test for xdp_master_redirect with bond not up

Add a selftest that reproduces the null-ptr-deref in
bond_rr_gen_slave_id() when XDP redirect targets a bond device in
round-robin mode that was never brought up. The test verifies the fix
by ensuring no crash occurs.

Test setup:
- bond0: active-backup mode, UP, with native XDP (enables
  bpf_master_redirect_enabled_key globally)
- bond1: round-robin mode, never UP
- veth1: slave of bond1, with generic XDP (XDP_TX)
- BPF_PROG_TEST_RUN with live frames triggers the redirect path

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260411005524.201200-3-jiayuan.chen@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>