Drop some headers from mm.h

mm.h doesn't use directly anything from mutex.h and backing-dev.h, so
remove them and add them back to files which need them.

Cross-compile tested on many configs and archs.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/fremap.c b/mm/fremap.c
index 95bcb564..37a93f8 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -5,7 +5,7 @@
  *
  * started by Ingo Molnar, Copyright (C) 2002, 2003
  */
-
+#include <linux/backing-dev.h>
 #include <linux/mm.h>
 #include <linux/swap.h>
 #include <linux/file.h>