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/fs/afs/write.c b/fs/afs/write.c
index bf0955d..9a849ad 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -8,7 +8,7 @@
  * as published by the Free Software Foundation; either version
  * 2 of the License, or (at your option) any later version.
  */
-
+#include <linux/backing-dev.h>
 #include <linux/slab.h>
 #include <linux/fs.h>
 #include <linux/pagemap.h>