sched/headers: Remove <linux/sched.h> from <linux/sched/mm.h>

sched/headers: Remove <linux/sched.h> from <linux/sched/mm.h>

The <linux/sched/mm.h> file is a self-contained header and users of
it either don't need <linux/sched.h> - or have already included it.

Include kernel.h and atomic.h.

This reduces the size of the header dependency graph.

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]>