selftests/bpf: add test for xdp_bonding xmit_hash_policy compat

selftests/bpf: add test for xdp_bonding xmit_hash_policy compat

Add a selftest to verify that changing xmit_hash_policy to vlan+srcmac
is rejected when a native XDP program is loaded on a bond in 802.3ad
mode.  Without the fix in bond_option_xmit_hash_policy_set(), the change
succeeds silently, creating an inconsistent state that triggers a kernel
WARNING in dev_xdp_uninstall() when the bond is torn down.

The test attaches native XDP to a bond0 (802.3ad, layer2+3), then
attempts to switch xmit_hash_policy to vlan+srcmac and asserts the
operation fails.  It also verifies the change succeeds after XDP is
detached, confirming the rejection is specific to the XDP-loaded state.

Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Link: https://patch.msgid.link/20260226080306.98766-3-jiayuan.chen@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>