drbd: fix test of unsigned in _drbd_fault_random()

rsp->count is unsigned so the test does not work.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Lars Ellenberg <drbd-dev@lists.linbit.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>