MDEV-16996 mariabackup --prepare does not use native AIO on Linux by default

extra/mariabackup/xtrabackup.cc

@@ -1216,7 +1216,7 @@ struct my_option xb_server_options[] =
    "Use native AIO if supported on this platform.",
    (G_PTR*) &srv_use_native_aio,
    (G_PTR*) &srv_use_native_aio, 0, GET_BOOL, NO_ARG,
-   FALSE, 0, 0, 0, 0, 0},
+   TRUE, 0, 0, 0, 0, 0},
   {"innodb_page_size", OPT_INNODB_PAGE_SIZE,
    "The universal page size of the database.",
    (G_PTR*) &innobase_page_size, (G_PTR*) &innobase_page_size, 0,