btrfs: use btrfs_fs_closing for background bg work

For both unused bg deletion and async balance work we'll happily run if
the fs is closing.  However I want to move these to their own worker
thread, and they can be long running jobs, so add a check to see if
we're closing and simply bail.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -1304,6 +1304,9 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
 	if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags))
 		return;
 
+	if (btrfs_fs_closing(fs_info))
+		return;
+
 	/*
 	 * Long running balances can keep us blocked here for eternity, so
 	 * simply skip deletion if we're unable to get the mutex.
@@ -1543,6 +1546,9 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 	if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags))
 		return;
 
+	if (btrfs_fs_closing(fs_info))
+		return;
+
 	if (!btrfs_should_reclaim(fs_info))
 		return;