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: NLinus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>

include/linux/sched/mm.h

 #ifndef _LINUX_SCHED_MM_H
 #define _LINUX_SCHED_MM_H
 
+#include <linux/kernel.h>
+#include <linux/atomic.h>
 #include <linux/sched.h>
 #include <linux/mm_types.h>
 #include <linux/gfp.h>