btrfs: fix WARNING in btrfs_select_ref_head()

This issue was found when testing in-band dedupe enospc behaviour,
sometimes run_one_delayed_ref() may fail for enospc reason, then
__btrfs_run_delayed_refs（）will return, but forget to add num_heads_read
back, which will trigger "WARN_ON(delayed_refs->num_heads_ready == 0)" in
btrfs_select_ref_head().

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -2647,7 +2647,10 @@ static noinline int __btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
 
 		btrfs_free_delayed_extent_op(extent_op);
 		if (ret) {
+			spin_lock(&delayed_refs->lock);
 			locked_ref->processing = 0;
+			delayed_refs->num_heads_ready++;
+			spin_unlock(&delayed_refs->lock);
 			btrfs_delayed_ref_unlock(locked_ref);
 			btrfs_put_delayed_ref(ref);
 			btrfs_debug(fs_info, "run_one_delayed_ref returned %d",