btrfs: always pass readahead state to defrag

Defrag ioctl passes readahead from the file, but autodefrag does not
have a file so the readahead state is allocated when needed.

The autodefrag loop in cleaner thread iterates over inodes so we can
simply provide an on-stack readahead state and will not need to allocate
it in btrfs_defrag_file(). The size is 32 bytes which is acceptable.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>