selftests/bpf: Fix massive output from test_maps

When stdout output from the selftests tool 'test_maps' gets redirected
into e.g file or pipe, then the output lines increase a lot (from 21
to 33949 lines). This is caused by the printf that happens before the
fork() call, and there are user-space buffered printf data that seems
to be duplicated into the forked process.

To fix this fflush() stdout before the fork loop in __run_parallel().

Fixes: 1a97cf1fe503 ("selftests/bpf: speedup test_maps")
Signed-off-by: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/159842985651.1050885.2154399297503372406.stgit@firesoul

tools/testing/selftests/bpf/test_maps.c

···
 	pid_t pid[tasks];
 	int i;
 
+	fflush(stdout);
+
 	for (i = 0; i < tasks; i++) {
 		pid[i] = fork();
 		if (pid[i] == 0) {