btrfs: Replace fs_info->flush_workers with btrfs_workqueue.

Replace the fs_info->submit_workers with the newly created
btrfs_workqueue.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Tested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 420328b..5b82b0b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2010,7 +2010,7 @@
 	btrfs_stop_workers(&fs_info->delayed_workers);
 	btrfs_stop_workers(&fs_info->caching_workers);
 	btrfs_stop_workers(&fs_info->readahead_workers);
-	btrfs_stop_workers(&fs_info->flush_workers);
+	btrfs_destroy_workqueue(fs_info->flush_workers);
 	btrfs_stop_workers(&fs_info->qgroup_rescan_workers);
 }
 
@@ -2483,9 +2483,8 @@
 	fs_info->delalloc_workers =
 		btrfs_alloc_workqueue("delalloc", flags, max_active, 2);
 
-	btrfs_init_workers(&fs_info->flush_workers, "flush_delalloc",
-			   fs_info->thread_pool_size, NULL);
-
+	fs_info->flush_workers =
+		btrfs_alloc_workqueue("flush_delalloc", flags, max_active, 0);
 
 	btrfs_init_workers(&fs_info->caching_workers, "cache",
 			   fs_info->thread_pool_size, NULL);
@@ -2560,14 +2559,13 @@
 	ret |= btrfs_start_workers(&fs_info->delayed_workers);
 	ret |= btrfs_start_workers(&fs_info->caching_workers);
 	ret |= btrfs_start_workers(&fs_info->readahead_workers);
-	ret |= btrfs_start_workers(&fs_info->flush_workers);
 	ret |= btrfs_start_workers(&fs_info->qgroup_rescan_workers);
 	if (ret) {
 		err = -ENOMEM;
 		goto fail_sb_buffer;
 	}
 	if (!(fs_info->workers && fs_info->delalloc_workers &&
-	      fs_info->submit_workers)) {
+	      fs_info->submit_workers && fs_info->flush_workers)) {
 		err = -ENOMEM;
 		goto fail_sb_buffer;
 	}