sched/walt: Access sched_boost_enabled with RCU protection

The task attributes embedded in schedtune cgroup controller must be
accessed from RCU critical section. All the attributes except
sched_boost_enabled have wrappers that honor this. Update
task_sched_boost() to do the same. Without this, the following
warning is observed.

[    0.004354] =============================
[    0.004361] WARNING: suspicious RCU usage
[    0.004374] 4.14.117-perf+ #272 Not tainted
[    0.004381] -----------------------------

[    0.004556] dump_stack+0xa8/0x100
[    0.004567] lockdep_rcu_suspicious+0x134/0x150
[    0.004576] task_sched_boost+0x8c/0xa8
[    0.004587] enqueue_task_fair+0x74/0x37c0
[    0.004596] activate_task+0xb0/0x300
[    0.004604] wake_up_new_task+0x1d4/0x4c0
[    0.004615] _do_fork+0x2dc/0x638
[    0.004623] kernel_thread+0x34/0x40
[    0.004634] rest_init+0x24/0x1f8
[    0.004646] start_kernel+0x424/0x4a0

Change-Id: Idba32e04f46487b46ebcf92d39f2c701caa316f4
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>