bpf: selftests: add CPUMAP/DEVMAP selftests for xdp frags

Verify compatibility checks attaching a XDP frags program to a
CPUMAP/DEVMAP

Acked-by: Toke Hoiland-Jorgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/d94b4d35adc1e42c9ca5004e6b2cdfd75992304d.1642758637.git.lorenzo@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>