io_uring: expose single mmap capability

After commit 75b28affdd we can get by with just a single mmap to
map both the sq and cq ring. However, userspace doesn't know that.

Add a features variable to io_uring_params, and notify userspace
that the kernel has this ability. This can then be used in liburing
(or in applications directly) to avoid the second mmap.

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

fs/io_uring.c

@@ -3391,6 +3391,8 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
 	p->cq_off.ring_entries = offsetof(struct io_rings, cq_ring_entries);
 	p->cq_off.overflow = offsetof(struct io_rings, cq_overflow);
 	p->cq_off.cqes = offsetof(struct io_rings, cqes);
+
+	p->features = IORING_FEAT_SINGLE_MMAP;
 	return ret;
 err:
 	io_ring_ctx_wait_and_kill(ctx);