selftests/bpf: verify kallsyms entries for token-loaded subprograms

Add a test that loads an XDP program with a global subprogram using a
BPF token from a user namespace, then verifies that both the main
program and the subprogram appear in /proc/kallsyms.

This exercises the bpf_prog_kallsyms_add() path for subprograms and
would have caught the missing aux->token copy in bpf_jit_subprogs().

load_kallsyms_local() filters out kallsyms with zero addresses.
For a process with limited capabilities to read kallsym addresses the
following sysctl variables have to be set to zero:
- /proc/sys/kernel/perf_event_paranoid
- /proc/sys/kernel/kptr_restrict
Set these variables using sysctl_set() utility function extracted from
unpriv_bpf_disabled.c to a separate c/header.
Since the test modifies global system state, mark it as serial.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260415-subprog-token-fix-v4-2-9bd000e8b068@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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