selftests/bpf: Add test for passing in uninit mtu_len

selftests/bpf: Add test for passing in uninit mtu_len

Add a small test to pass an uninitialized mtu_len to the bpf_check_mtu()
helper to probe whether the verifier rejects it under !CAP_PERFMON.

  # ./vmtest.sh -- ./test_progs -t verifier_mtu
  [...]
  ./test_progs -t verifier_mtu
  [    1.414712] tsc: Refined TSC clocksource calibration: 3407.993 MHz
  [    1.415327] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x311fcd52370, max_idle_ns: 440795242006 ns
  [    1.416463] clocksource: Switched to clocksource tsc
  [    1.429842] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.430283] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  #510/1   verifier_mtu/uninit/mtu: write rejected:OK
  #510     verifier_mtu:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241021152809.33343-5-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>