io_uring: mark ->work uninitialised after cleanup

Remove REQ_F_WORK_INITIALIZED after io_req_clean_work(). That's a cold
path but is safer for those using io_req_clean_work() out of
*dismantle_req()/*io_free(). And for the same reason zero work.fs

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