io_uring: keep io_req_free_batch() call locality

Don't do a function call (io_dismantle_req()) in the middle and place it
to near other function calls, otherwise may lead to excessive register
spilling.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>