btrfs: clean up the check for uncompressed ranges in submit_one_async_extent

Instead of checking for a NULL !pages and explaining this with a cryptic
comment, just check the compression type for BTRFS_COMPRESS_NONE to make
the check self-explanatory.

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>