selftests/bpf: Fix fclose/pclose mismatch in test_progs

Make sure to use pclose() to properly close the pipe opened by popen().

Fixes: 81f77fd ("bpf: add selftest for stackmap with BPF_F_STACK_BUILD_ID")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20211026143409.42666-1-andrea.righi@canonical.com

tools/testing/selftests/bpf/test_progs.c

@@ -382,7 +382,7 @@ int extract_build_id(char *build_id, size_t size)
 
 	if (getline(&line, &len, fp) == -1)
 		goto err;
-	fclose(fp);
+	pclose(fp);
 
 	if (len > size)
 		len = size;
@@ -391,7 +391,7 @@ int extract_build_id(char *build_id, size_t size)
 	free(line);
 	return 0;
 err:
-	fclose(fp);
+	pclose(fp);
 	return -1;
 }