param_shmem: do fsync after param write

src/modules/systemlib/param/param_shmem.c

@@ -857,7 +857,16 @@ param_save_default(void)
 		goto exit;
 	}
 
+	// After writing the file, also do a fsync to prevent loosing params if power is cut.
+	res = fsync(fd);
+	if (res != 0) {
+		PX4_ERR("failed to do fsync: %s", strerror(errno));
+		goto exit;
+	}
+
 	PARAM_CLOSE(fd);
+
+
 	fd = -1;
 
 exit: