fs/aio: aio_wq isn't used in memory reclaim path

aio_wq isn't used during memory reclaim.  Convert to alloc_workqueue()
without WQ_MEM_RECLAIM.  It's possible to use system_wq but given that
the number of work items is determined from userland and the work item
may block, enforcing strict concurrency limit would be a good idea.

Also, move fput_work to system_wq so that aio_wq is used soley to
throttle the max concurrency of aio work items and fput_work doesn't
interact with other work items.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: linux-aio@kvack.org

fs/aio.c

@@ -85,7 +85,7 @@ static int __init aio_setup(void)
 	kiocb_cachep = KMEM_CACHE(kiocb, SLAB_HWCACHE_ALIGN|SLAB_PANIC);
 	kioctx_cachep = KMEM_CACHE(kioctx,SLAB_HWCACHE_ALIGN|SLAB_PANIC);
 
-	aio_wq = create_workqueue("aio");
+	aio_wq = alloc_workqueue("aio", 0, 1);	/* used to limit concurrency */
 	abe_pool = mempool_create_kmalloc_pool(1, sizeof(struct aio_batch_entry));
 	BUG_ON(!aio_wq || !abe_pool);
 
@@ -569,7 +569,7 @@ static int __aio_put_req(struct kioctx *ctx, struct kiocb *req)
 		spin_lock(&fput_lock);
 		list_add(&req->ki_list, &fput_head);
 		spin_unlock(&fput_lock);
-		queue_work(aio_wq, &fput_work);
+		schedule_work(&fput_work);
 	} else {
 		req->ki_filp = NULL;
 		really_put_req(ctx, req);