FIXUP: sched/tune: add fixes missing from a previous patch

The previous patch:

  e7ce26f - FIXUP: sched/tune: fix accounting for runnable tasks

squashed together patches of a series to fix SchedTune's accounting
issues. However, in the consolidation and cleanup of the series to merge
in the Android Common Kernel, we somehow missed a couple of important
changes:
1) the schedtune_exit function is not more required, because e7ce26f
   fixes accounting of exiting tasks in a different way
2) the schedtune_initialized flag was not set at the end of
   scheddtune_init_cgroup() thus failing to enabled SchedTune at boot.

This patch thus is to be considered an integration of e7ce26f.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
[jstultz: Cherry-picked from android-3.18. It should be noted that
some of this patch was already applied in the 4.4 patches (schedtune_exit
doesn't exist for example), but this patch just ensures things are totally
synced up]
Signed-off-by: John Stultz <john.stultz@linaro.org>

kernel/sched/tune.c

@@ -715,7 +715,7 @@ schedtune_css_free(struct cgroup_subsys_state *css)
 struct cgroup_subsys schedtune_cgrp_subsys = {
 	.css_alloc	= schedtune_css_alloc,
 	.css_free	= schedtune_css_free,
-//	.allow_attach   = schedtune_allow_attach,
+	.allow_attach   = schedtune_allow_attach,
 	.can_attach     = schedtune_can_attach,
 	.cancel_attach  = schedtune_cancel_attach,
 	.legacy_cftypes	= files,
@@ -736,6 +736,8 @@ schedtune_init_cgroups(void)
 
 	pr_info("schedtune: configured to support %d boost groups\n",
 		BOOSTGROUPS_COUNT);
+
+	schedtune_initialized = true;
 }
 
 #else /* CONFIG_CGROUP_SCHEDTUNE */