io_uring/rsrc: keep one global dummy_ubuf

We set empty registered buffers to dummy_ubuf as an optimisation.
Currently, we allocate the dummy entry for each ring, whenever we can
simply have one global instance.

We're casting out const on assignment, it's fine as we're not going to
change the content of the dummy, the constness gives us an extra layer
of protection if sth ever goes wrong.

Signed-off-by: Pavel Begunkov <[email protected]>
Link: https://lore.kernel.org/r/e4a96dda35ab755914bc43f6781bba0df97ac489.1691757663.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <[email protected]>

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