io_uring: hide uring_fd in ctx

req->ring_fd and req->ring_file are used only during the prep stage
during submission, which is is protected by mutex. There is no need
to store them per-request, place them in ctx.

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