Move some code around to better separate front/backend

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index e7107d4..9af278a 100644
--- a/server.c
+++ b/server.c
@@ -346,7 +346,7 @@
 	spdu.jobs = cpu_to_le32(thread_number);
 	fio_net_send_cmd(server_fd, FIO_NET_CMD_START, &spdu, sizeof(spdu), 0);
 
-	ret = exec_run();
+	ret = fio_backend();
 
 	epdu.error = ret;
 	fio_net_send_cmd(server_fd, FIO_NET_CMD_STOP, &epdu, sizeof(epdu), 0);
@@ -390,7 +390,7 @@
 
 	fio_net_send_simple_cmd(server_fd, FIO_NET_CMD_START, 0, NULL);
 
-	ret = exec_run();
+	ret = fio_backend();
 	fio_server_send_quit_cmd();
 	reset_fio_state();
 	return ret;