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>

include/uapi/linux/io_uring/query.h

@@ -36,6 +36,9 @@ struct io_uring_query_opcode {
 	__u64	enter_flags;
 	/* Bitmask of all supported IOSQE_* flags */
 	__u64	sqe_flags;
+	/* The number of available query opcodes */
+	__u32	nr_query_opcodes;
+	__u32	__pad;
 };
 
 #endif