io_uring/query: introduce zcrx query

io_uring/query: introduce zcrx query

Add a new query type IO_URING_QUERY_ZCRX returning the user some basic
information about the interface, which includes allowed flags for areas
and registration and supported IORING_REGISTER_ZCRX_CTRL subcodes.

There is also a chicken-egg problem with user provided refill queue
memory, where offsets and size information is returned after
registration, but to properly allocate memory you need to know it
beforehand, which is why the userspace currently has to guess the RQ
headers size and severely overestimates it. Return the size information.
It's split into "size" and "alignment" fields because for default
placement modes the user is interested in the aligned size, however if
it gets support for more flexible placement, it'll need to only know the
actual header size.

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