io_uring: minor refactoring for some tw handlers

Get rid of some useless local variables

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7798327b684b7015f7e4300420142ddfcd317297.1650056133.git.asml.silence@gmail.com


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