btrfs: make cow_file_range_async take btrfs_inode

It only uses vfs inode for assigning it to the async_chunk function.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>