btrfs: remove do_list variable at btrfs_set_delalloc_extent()

The "do_list" variable is only used once, plus its name/meaning is a bit
confusing, so remove it and directory use btrfs_is_free_space_inode().

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>