backend: ensure that server side releases/frees shm segment on exit

We were leaking one for every job run.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index 8b91d26..c859ad7 100644
--- a/server.c
+++ b/server.c
@@ -550,6 +550,7 @@
 	}
 
 	ret = fio_backend();
+	free_threads_shm();
 	_exit(ret);
 }