Btrfs: handle EAGAIN case properly in btrfs_drop_snapshot()

commit 90515e7f upstream.

We may return early in btrfs_drop_snapshot(), we shouldn't
call btrfs_std_err() for this case, fix it.
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/btrfs/extent-tree.c

@@ -6725,7 +6725,7 @@ void btrfs_drop_snapshot(struct btrfs_root *root,
 	 */
 	if (root_dropped == false)
 		btrfs_add_dead_root(root);
-	if (err)
+	if (err && err != -EAGAIN)
 		btrfs_std_error(root->fs_info, err);
 	return;
 }