selftests/bpf: Cover pseudo-BTF ksym log masking

selftests/bpf: Cover pseudo-BTF ksym log masking

Add verifier_unpriv coverage for a raw socket-filter load of the
bpf_prog_active typed ksym. The test verifies that the unprivileged load
remains accepted and that the verbose verifier log prints the ldimm64
immediate as 0x0 instead of exposing a nonzero kernel address.

Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
Link: https://lore.kernel.org/r/20260623-f01-13-pseudo-btf-id-cap-bpf-v2-2-a190ebb8f3e2@mails.tsinghua.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>