drbd: fix if(); found by kbuild test robot

Recently introduced al_begin_io_nonblock() was returning -EBUSY,
even when it should return -EWOULDBLOCK.

Impact:
A few spurious wake_up() calls in prepare_al_transaction_nonblock().
Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_actlog.c

@@ -389,7 +389,7 @@ int drbd_al_begin_io_nonblock(struct drbd_conf *mdev, struct drbd_interval *i)
 		if (unlikely(tmp != NULL)) {
 			struct bm_extent  *bm_ext = lc_entry(tmp, struct bm_extent, lce);
 			if (test_bit(BME_NO_WRITES, &bm_ext->flags)) {
-				if (!test_and_set_bit(BME_PRIORITY, &bm_ext->flags));
+				if (!test_and_set_bit(BME_PRIORITY, &bm_ext->flags))
 					return -EBUSY;
 				return -EWOULDBLOCK;
 			}