Btrfs: stop all workers before cleaning up roots

Dave reported a panic because the extent_root->commit_root was NULL in the
caching kthread.  That is because we just unset it in free_root_pointers, which
is not the correct thing to do, we have to either wait for the caching kthread
to complete or hold the extent_commit_sem lock so we know the thread has exited.
This patch makes the kthreads all stop first and then we do our cleanup.  This
should fix the race.  Thanks,

Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/disk-io.c

@@ -3514,13 +3514,13 @@ int close_ctree(struct btrfs_root *root)
 
 	btrfs_free_block_groups(fs_info);
 
-	free_root_pointers(fs_info, 1);
+	btrfs_stop_all_workers(fs_info);
 
 	del_fs_roots(fs_info);
 
-	iput(fs_info->btree_inode);
+	free_root_pointers(fs_info, 1);
 
-	btrfs_stop_all_workers(fs_info);
+	iput(fs_info->btree_inode);
 
 #ifdef CONFIG_BTRFS_FS_CHECK_INTEGRITY
 	if (btrfs_test_opt(root, CHECK_INTEGRITY))