selftests/bpf: xdp_metadata: Check XDP_REDIRCT support for dev-bound progs

Improve xdp_metadata bpf selftest in order to check it is possible for a
XDP dev-bound program to perform XDP_REDIRECT into a DEVMAP but it is still
not allowed to attach a XDP dev-bound program to a DEVMAP entry.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>