io_uring/mock: add cmd using vectored regbufs

There is a command api allowing to import vectored registered buffers,
add a new mock command that uses the feature and simply copies the
specified registered buffer into user space or vice versa.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/229a113fd7de6b27dbef9567f7c0bf4475c9017d.1750599274.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>