selftests/bpf: Tests using bpf_check_mtu BPF-helper input mtu_len param

Add tests that use mtu_len as input parameter in BPF-helper
bpf_check_mtu().

The BPF-helper is avail from both XDP and TC context. Add two tests
per context, one that tests below MTU and one that exceeds the MTU.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/161521556358.3515614.5915221479709358964.stgit@firesoul