Fix the cgroup v2 thread path

The name for the file with thread IDs is "tasks" in the v1 hierarchy and
"cgroup.threads" in the v2 hierarchy.

References:
* https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v1/cgroups.html
* https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html

Bug: 213617178
Fixes: 82b72a566761 ("libprocessgroup: Add support for task profiles")
Change-Id: I87d67edeb12803cb83486be8f1b1b56a3f275a9d
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/libprocessgroup/cgroup_map.cpp b/libprocessgroup/cgroup_map.cpp
index 8c00326..468d796 100644
--- a/libprocessgroup/cgroup_map.cpp
+++ b/libprocessgroup/cgroup_map.cpp
@@ -49,7 +49,7 @@
 
 static constexpr const char* CGROUP_PROCS_FILE = "/cgroup.procs";
 static constexpr const char* CGROUP_TASKS_FILE = "/tasks";
-static constexpr const char* CGROUP_TASKS_FILE_V2 = "/cgroup.tasks";
+static constexpr const char* CGROUP_TASKS_FILE_V2 = "/cgroup.threads";
 
 uint32_t CgroupController::version() const {
     CHECK(HasValue());