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>

tools/testing/selftests/bpf/prog_tests/unpriv_bpf_disabled.c

@@ -8,6 +8,7 @@
 
 #include "cap_helpers.h"
 #include "bpf_util.h"
+#include "sysctl_helpers.h"
 
 /* Using CAP_LAST_CAP is risky here, since it can get pulled in from
  * an old /usr/include/linux/capability.h and be < CAP_BPF; as a result
@@ -36,26 +37,6 @@ static void process_perfbuf(void *ctx, int cpu, void *data, __u32 len)
 		got_perfbuf_val = *(__u32 *)data;
 }
 
-static int sysctl_set(const char *sysctl_path, char *old_val, const char *new_val)
-{
-	int ret = 0;
-	FILE *fp;
-
-	fp = fopen(sysctl_path, "r+");
-	if (!fp)
-		return -errno;
-	if (old_val && fscanf(fp, "%s", old_val) <= 0) {
-		ret = -ENOENT;
-	} else if (!old_val || strcmp(old_val, new_val) != 0) {
-		fseek(fp, 0, SEEK_SET);
-		if (fprintf(fp, "%s", new_val) < 0)
-			ret = -errno;
-	}
-	fclose(fp);
-
-	return ret;
-}
-
 static void test_unpriv_bpf_disabled_positive(struct test_unpriv_bpf_disabled *skel,
 					      __u32 prog_id, int prog_fd, int perf_fd,
 					      char **map_paths, int *map_fds)