sched/headers: Move the NOHZ APIs from <linux/sched.h> to <linux/sched/nohz.h>

sched/headers: Move the NOHZ APIs from <linux/sched.h> to <linux/sched/nohz.h>

There's a number of NOHZ/dyntics related functionality in <linux/sched.h>,
but only a handful of timer files are making use of them.

Move them into their own header. This better documents these APIs
and unclutters <linux/sched.h>.

Acked-by: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>