btrfs: Document logic regarding inode in async_cow_submit

Add a comment explaining when ->inode could be NULL and why we always
perform the ->async_delalloc_pages modification.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1165,6 +1165,12 @@ static noinline void async_cow_submit(struct btrfs_work *work)
 	    5 * SZ_1M)
 		cond_wake_up_nomb(&fs_info->async_submit_wait);
 
+	/*
+	 * ->inode could be NULL if async_cow_start has failed to compress,
+	 * in which case we don't have anything to submit, yet we need to
+	 * always adjust ->async_delalloc_pages as its paired with the init
+	 * happening in cow_file_range_async
+	 */
 	if (async_cow->inode)
 		submit_compressed_extents(async_cow);
 }