drbd: simplify condition in drbd_may_do_local_read()

fold
	if (x >= (N+1))
		return 0;
	if (x < N)
		return 0;
into
	if (x != N)
		return 0;

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