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>