sched/tune: fix suspicious RCU usage warning

Below warning is observed during boot when lockdep is enabled.
Fix it by adding RCU lock around sched_boost_enabled usage.

[    0.158789] WARNING: suspicious RCU usage
[    0.158819] include/linux/cgroup.h:477 \
		suspicious rcu_dereference_check() usage!
[    0.158935] Call trace:
[    0.158948]  dump_backtrace+0x0/0x190
[    0.158958]  show_stack+0x20/0x30
[    0.158970]  dump_stack+0xe8/0x13c
[    0.158981]  lockdep_rcu_suspicious+0x128/0x150
[    0.158994]  task_sched_boost+0x90/0xa8
[    0.159005]  pick_next_task_fair+0x7f8/0x9e0
[    0.159016]  __schedule+0x92c/0x1028
[    0.159026]  schedule_idle+0x24/0x40
[    0.159038]  do_idle+0x270/0x2a8
[    0.159049]  cpu_startup_entry+0x24/0x28
[    0.159060]  secondary_start_kernel+0x168/0x178.

Change-Id: Id1d95744aa91d8aafee9ec0c413468b5ee18f0db
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>