selftests/bpf: Add per worker cgroup suffix

This patch make each worker use a unique cgroup base directory, thus
allowing tests that uses cgroups to run concurrently.

Signed-off-by: Yucong Sun <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/cgroup_helpers.c

···
 #include <fcntl.h>
 #include <unistd.h>
 #include <ftw.h>
+#include <unistd.h>
 
 #include "cgroup_helpers.h"
 
···
 #define CGROUP_MOUNT_DFLT		"/sys/fs/cgroup"
 #define NETCLS_MOUNT_PATH		CGROUP_MOUNT_DFLT "/net_cls"
 #define CGROUP_WORK_DIR			"/cgroup-test-work-dir"
-
 #define format_cgroup_path(buf, path) \
-	snprintf(buf, sizeof(buf), "%s%s%s", CGROUP_MOUNT_PATH, \
-		 CGROUP_WORK_DIR, path)
+	snprintf(buf, sizeof(buf), "%s%s%d%s", CGROUP_MOUNT_PATH, \
+	CGROUP_WORK_DIR, getpid(), path)
 
 #define format_classid_path(buf)				\
 	snprintf(buf, sizeof(buf), "%s%s", NETCLS_MOUNT_PATH,	\