drivers/block/aoe/aoedev: NULL check is not needed for mempool_destroy

mempool_destroy has taken the null pointer into account. So it is safe
to remove the null check.
Signed-off-by: Nzhong jiang <zhongjiang@huawei.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/block/aoe/aoedev.c

@@ -284,8 +284,8 @@ freedev(struct aoedev *d)
 	e = t + d->ntargets;
 	for (; t < e && *t; t++)
 		freetgt(d, *t);
-	if (d->bufpool)
-		mempool_destroy(d->bufpool);
+
+	mempool_destroy(d->bufpool);
 	skbpoolfree(d);
 	minor_free(d->sysminor);