sched/sysctl: Fix attributes of some extern declarations

The definition of sysctl_sched_migration_cost, sysctl_sched_nr_migrate
and sysctl_sched_time_avg includes the attribute const_debug. This
attribute is not part of the extern declaration of these variables in
include/linux/sched/sysctl.h, while it is in kernel/sched/sched.h,
and as a result Clang generates warnings like this:

  kernel/sched/sched.h:1618:33: warning: section attribute is specified on redeclared variable [-Wsection]
  extern const_debug unsigned int sysctl_sched_time_avg;
                                ^
  ./include/linux/sched/sysctl.h:42:21: note: previous declaration is here
  extern unsigned int sysctl_sched_time_avg;

The header only declares the variables when CONFIG_SCHED_DEBUG is defined,
therefore it is not necessary to duplicate the definition of const_debug.
Instead we can use the attribute __read_mostly, which is the expansion of
const_debug when CONFIG_SCHED_DEBUG=y is set.

Signed-off-by: Matthias Kaehlcke <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Cc: Douglas Anderson <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Shile Zhang <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Matthias Kaehlcke

include/linux/sched/sysctl.h

@@ -37,9 +37,9 @@ extern unsigned int sysctl_numa_balancing_scan_period_max;
 extern unsigned int sysctl_numa_balancing_scan_size;
 
 #ifdef CONFIG_SCHED_DEBUG
-extern unsigned int sysctl_sched_migration_cost;
-extern unsigned int sysctl_sched_nr_migrate;
-extern unsigned int sysctl_sched_time_avg;
+extern __read_mostly unsigned int sysctl_sched_migration_cost;
+extern __read_mostly unsigned int sysctl_sched_nr_migrate;
+extern __read_mostly unsigned int sysctl_sched_time_avg;
 
 int sched_proc_update_handler(struct ctl_table *table, int write,
 		void __user *buffer, size_t *length,