sched/headers: Prepare for new header dependencies before moving code to <linux/sched/mm.h>

We are going to split <linux/sched/mm.h> out of <linux/sched.h>, which
will have to be picked up from other headers and a couple of .c files.

Create a trivial placeholder <linux/sched/mm.h> file that just
maps to <linux/sched.h> to make this patch obviously correct and
bisectable.

The APIs that are going to be moved first are:

   mm_alloc()
   __mmdrop()
   mmdrop()
   mmdrop_async_fn()
   mmdrop_async()
   mmget_not_zero()
   mmput()
   mmput_async()
   get_task_mm()
   mm_access()
   mm_release()

Include the new header in the files that are going to need it.

Acked-by: Linus 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: Ingo Molnar <mingo@kernel.org>

fs/proc/array.c

@@ -60,6 +60,7 @@
 #include <linux/tty.h>
 #include <linux/string.h>
 #include <linux/mman.h>
+#include <linux/sched/mm.h>
 #include <linux/proc_fs.h>
 #include <linux/ioport.h>
 #include <linux/uaccess.h>

fs/proc/base.c

@@ -86,6 +86,7 @@
 #include <linux/fs_struct.h>
 #include <linux/slab.h>
 #include <linux/sched/autogroup.h>
+#include <linux/sched/mm.h>
 #include <linux/flex_array.h>
 #include <linux/posix-timers.h>
 #ifdef CONFIG_HARDWALL

fs/proc/task_mmu.c

@@ -11,6 +11,7 @@
 #include <linux/mempolicy.h>
 #include <linux/rmap.h>
 #include <linux/swap.h>
+#include <linux/sched/mm.h>
 #include <linux/swapops.h>
 #include <linux/mmu_notifier.h>
 #include <linux/page_idle.h>

fs/proc/task_nommu.c

@@ -7,6 +7,8 @@
 #include <linux/ptrace.h>
 #include <linux/slab.h>
 #include <linux/seq_file.h>
+#include <linux/sched/mm.h>
+
 #include "internal.h"
 
 /*