Adding selftest for BPF-helper bpf_check_mtu(). Making sure
it can be used from both XDP and TC.

V16:
 - Fix 'void' function definition

V11:
 - Addresse nitpicks from Andrii Nakryiko

V10:
 - Remove errno non-zero test in CHECK_ATTR()
 - Addresse comments from Andrii Nakryiko
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/161287791989.790810.13612620012522164562.stgit@firesoul