sched: optimize sched_clock() a bit

sched_clock() uses cycles_2_ns() needlessly - which is an irq-disabling
variant of __cycles_2_ns().

Most of the time sched_clock() is called with irqs disabled already.
The few places that call it with irqs enabled need to be updated.

Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/tsc.c

···
 	rdtscll(this_offset);
 
 	/* return the value in ns */
-	return cycles_2_ns(this_offset);
+	return __cycles_2_ns(this_offset);
 }
 
 /* We need to define a real function for sched_clock, to override the