selftests/bpf: Test bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) when transport_header is...

selftests/bpf: Test bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) when transport_header is not set

Add a test to check that bpf_skb_check_mtu(BPF_MTU_CHK_SEGS) is
rejected (-EINVAL) if skb->transport_header is not set. The test
needs to lower the MTU of the loopback device. Thus, take this
opportunity to run the test in a netns by adding "ns_" to the test
name. The "serial_" prefix can then be removed.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20251112232331.1566074-2-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>