io_uring: add missing item types for splice request

Splice is like read/write and should grab current->nsproxy, denoted by
IO_WQ_WORK_FILES as it refers to current->files as well

Signed-off-by: Bing-Jhong Billy Jheng <billy@starlabs.sg>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>