selftests/bpf: Add test case for different expected_attach_type

selftests/bpf: Add test case for different expected_attach_type

Add a small test case which adds two programs - one calling the other
through a tailcall - and check that BPF rejects them in case of different
expected_attach_type values:

  # ./vmtest.sh -- ./test_progs -t xdp_devmap
  [...]
  #641/1   xdp_devmap_attach/DEVMAP with programs in entries:OK
  #641/2   xdp_devmap_attach/DEVMAP with frags programs in entries:OK
  #641/3   xdp_devmap_attach/Verifier check of DEVMAP programs:OK
  #641/4   xdp_devmap_attach/DEVMAP with programs in entries on veth:OK
  #641     xdp_devmap_attach:OK
  Summary: 2/4 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20250926171201.188490-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>