io_uring: refactor io_submit_flush_completions

Don't init req_batch before we actually need it. Also, add a small clean
up for req declaration.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/ad85512e12bd3a20d521e9782750300970e5afc8.1624739600.git.asml.silence@gmail.com


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