drbd: removed duplicated #includes

drbd/drbd_receiver.c: linux/mm.h is included more than once.
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/drbd/drbd_receiver.c

@@ -42,7 +42,6 @@
 #include <linux/unistd.h>
 #include <linux/vmalloc.h>
 #include <linux/random.h>
-#include <linux/mm.h>
 #include <linux/string.h>
 #include <linux/scatterlist.h>
 #include "drbd_int.h"