sched: avoid using ilog2() in sched.h

<linux/sched.h> indirectly via cpumask.h path includes the ilog2.h header
to calculate ilog2(TASK_REPORT_MAX). The following patches drops sched.h
dependency on cpumask.h, and to have a successful build, the header has to
be included explicitly.

sched.h is a frequently included header, and it's better to keep the
dependency list as small as possible. So, instead of including ilog2.h
for a single BUILD_BUG_ON() check, the same check may be implemented by
taking exponent of the other part of equation.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Yury Norov <[e-mail]>
Cc: Amit Daniel Kachhap <[e-mail]>
Cc: Anna-Maria Behnsen <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Daniel Lezcano <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Juri Lelli <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Paul E. McKenney <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Rafael J. Wysocki <[e-mail]>
Cc: Rasmus Villemoes <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ulf Hansson <[e-mail]>
Cc: Vincent Guittot <[e-mail]>
Cc: Viresh Kumar <[e-mail]>
Cc: Yury Norov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/sched.h

···
 {
 	static const char state_char[] = "RSDTtXZPI";
 
-	BUILD_BUG_ON(1 + ilog2(TASK_REPORT_MAX) != sizeof(state_char) - 1);
+	BUILD_BUG_ON(TASK_REPORT_MAX * 2 != 1 << (sizeof(state_char) - 1));
 
 	return state_char[state];
 }