drbd: use kzalloc and memset(,0,) to start with clean buffers in drbd_nl

Make sure we start with clean buffers to not accidentally send garbage
back to userspace. Note: has not been observed; but just in case.

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