btrfs: Remove unused parameter from check_direct_IO

Introduced by 5a5f79b5

 ("Btrfs: allow unaligned DIO") and never
used. The buffered fallback from unaligned DIO works as expected.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>