sched/clock: Disable interrupts when calling generic_sched_clock_init()

sched_clock_init() used be called early during boot when interrupts were
still disabled. After the recent changes to utilize sched clock early the
sched_clock_init() call happens when interrupts are already enabled, which
triggers the following warning:

WARNING: CPU: 0 PID: 0 at kernel/time/sched_clock.c:180 sched_clock_register+0x44/0x278
[<c001a13c>] (warn_slowpath_null) from [<c052367c>] (sched_clock_register+0x44/0x278)
[<c052367c>] (sched_clock_register) from [<c05238d8>] (generic_sched_clock_init+0x28/0x88)
[<c05238d8>] (generic_sched_clock_init) from [<c0521a00>] (sched_clock_init+0x54/0x74)
[<c0521a00>] (sched_clock_init) from [<c0519c18>] (start_kernel+0x310/0x3e4)
[<c0519c18>] (start_kernel) from [<00000000>] (  (null))

Disable IRQs for the duration of generic_sched_clock_init().

Fixes: 857baa87b6 ("sched/clock: Enable sched clock early")
Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: steven.sistare@oracle.com
Cc: daniel.m.jordan@oracle.com
Link: https://lkml.kernel.org/r/20180730135252.24599-1-pasha.tatashin@oracle.com

kernel/sched/clock.c

@@ -452,7 +452,9 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
 void __init sched_clock_init(void)
 {
 	static_branch_inc(&sched_clock_running);
+	local_irq_disable();
 	generic_sched_clock_init();
+	local_irq_enable();
 }
 
 u64 sched_clock_cpu(int cpu)