btrfs: move forward declarations to the beginning of extent_io.h

There are two forward declarations deep in extent_io.h, move them
to the beginning and remove the duplicate one.

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