io_uring: fix fs cleanup on cqe overflow

If completion queue overflow occurs, __io_cqring_fill_event() will
update req->cflags, which is in a union with req->work and happens to
be aliased to req->work.fs. Following io_free_req() ->
io_req_work_drop_env() may get a bunch of different problems (miscount
fs->users, segfault, etc) on cleaning @fs.

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