virStorageBackendFileSystemMount: prefer strdup over virAsprintf

* src/storage/storage_backend_fs.c (virStorageBackendFileSystemMount):
Use virAsprintf only when needed.  In this case, strdup works fine.

src/storage/storage_backend_fs.c

@@ -376,7 +376,7 @@ virStorageBackendFileSystemMount(virStoragePoolObjPtr pool) {
 
     if (pool->def->type == VIR_STORAGE_POOL_NETFS) {
         if (pool->def->source.format == VIR_STORAGE_POOL_NETFS_GLUSTERFS) {
-            if (virAsprintf(&options, "direct-io-mode=1") == -1) {
+            if ((options = strdup("direct-io-mode=1")) == NULL) {
                 virReportOOMError();
                 return -1;
             }