io_uring/rsrc: replace reg buffer bit field with flags

io_uring/rsrc: replace reg buffer bit field with flags

I'll need a flag in the registered buffer struct for dmabuf work, and
it'll be more convenient to have a flags field rather than bit fields,
especially for io_mapped_ubuf initialisation.

We might want to add more flags in the future as well. For example, it
might be useful for debugging and potentially optimisations to split out
a flag indicating the shape of the buffer to gate iov_iter_advance()
walks vs bit/mask arithmetics. It can also be combined with the
direction mask field.

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