Btrfs: handle EAGAIN case properly in btrfs_drop_snapshot()

We may return early in btrfs_drop_snapshot(), we shouldn't
call btrfs_std_err() for this case, fix it.

Cc: stable@vger.kernel.org
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>

fs/btrfs/extent-tree.c

@@ -7835,7 +7835,7 @@ out:
 	 */
 	if (!for_reloc && root_dropped == false)
 		btrfs_add_dead_root(root);
-	if (err)
+	if (err && err != -EAGAIN)
 		btrfs_std_error(root->fs_info, err);
 	return err;
 }