drbd: Replace 0 with the more meaningful GFP_NOWAIT

GFP_NOWAIT has a value of 0. I.e. functionality not changed.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>