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>

fs/btrfs/inode.c

@@ -1206,8 +1206,7 @@ static void submit_one_async_extent(struct async_chunk *async_chunk,
 	}
 	lock_extent(io_tree, start, end, NULL);
 
-	/* We have fall back to uncompressed write */
-	if (!async_extent->pages) {
+	if (async_extent->compress_type == BTRFS_COMPRESS_NONE) {
 		submit_uncompressed_range(inode, async_extent, locked_page);
 		goto done;
 	}