selftests/bpf: Allow auto port binding for bpf nf

Allow auto port binding for bpf nf test to avoid binding conflict.

./test_progs -a bpf_nf
24/1    bpf_nf/xdp-ct:OK
24/2    bpf_nf/tc-bpf-ct:OK
24/3    bpf_nf/alloc_release:OK
24/4    bpf_nf/insert_insert:OK
24/5    bpf_nf/lookup_insert:OK
24/6    bpf_nf/set_timeout_after_insert:OK
24/7    bpf_nf/set_status_after_insert:OK
24/8    bpf_nf/change_timeout_after_alloc:OK
24/9    bpf_nf/change_status_after_alloc:OK
24/10   bpf_nf/write_not_allowlisted_field:OK
24      bpf_nf:OK
Summary: 1/10 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20250227142646.59711-3-jiayuan.chen@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>