io_uring: move pinning out of io_req_local_work_add

Move ctx pinning from io_req_local_work_add() to the caller, looks
better and makes working with the code a bit easier.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/49c0dbed390b0d6d04cb942dd3592879fd5bfb1b.1680782017.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>