io_uring: allow empty slots for reg buffers

Allow empty reg buffer slots any request using which should fail. This
allows users to not register all buffers in advance, but do it lazily
and/or on demand via updates. That is achieved by setting iov_base and
iov_len to zero for registration and/or buffer updates. Empty buffer
can't have a non-zero tag.

Implementation details: to not add extra overhead to io_import_fixed(),
create a dummy buffer crafted to fail any request using it, and set it
to all empty buffer slots.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7e95e4d700082baaf010c648c72ac764c9cc8826.1619611868.git.asml.silence@gmail.com


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