btrfs: rename cow_file_range_async to run_delalloc_compressed

cow_file_range_async is only used for compressed writeback.  Rename it
to run_delalloc_compressed, which also fits in with run_delalloc_nocow
and run_delalloc_zoned.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>