io_uring/query: return number of available queries

io_uring/query: return number of available queries

It's useful to know which query opcodes are available. Extend the
structure and return that. It's a trivial change, and even though it can
be painlessly extended later, it'd still require adding a v2 of the
structure.

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