io_uring: clean *local_work_add var naming

if (!first) { ... }

While it reads as do something if it's not the first entry, it does
exactly the opposite because "first" here is a pointer to the first
entry. Remove the confusion by renaming it into "head".

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3b8be483b52f58a524185bb88694b8a268e7e85d.1705438669.git.asml.silence@gmail.com


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