libaio: Add hint on libaio engine init failing

If io_queue_init() fails, it's likely due to a lack of
aio request resources.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/libaio.c b/engines/libaio.c
index eea8f55..7ef3ff8 100644
--- a/engines/libaio.c
+++ b/engines/libaio.c
@@ -194,6 +194,7 @@
 	err = io_queue_init(td->o.iodepth, &ld->aio_ctx);
 	if (err) {
 		td_verror(td, -err, "io_queue_init");
+		log_err("fio: check /proc/sys/fs/aio-max-nr\n");
 		free(ld);
 		return 1;
 	}