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>