selftests/bpf: Test XDP bonding nest and unwind

Modify the test to check that enslaving a bond slave with a XDP program
is now allowed.

Extend attach test to exercise the program unwinding in bond_xdp_set and
add a new test for loading XDP program on doubly nested bond device to
verify that static key incr/decr is correct.

Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>