block: kill DIO_MULTI_BIO

Now __blkdev_direct_IO() serves only multi-bio I/O, thus remove
not used anymore single bio refcounting optimisations.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/88eb488aae9ed4852a30f3a7132f296f56e43b80.1635337135.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>