fs: move memset to upper lever for statfs

if struct statfs add new members, such as f_fsid, no additional code
changes are required.

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

libs/libc/userfs/lib_userfs.c

@@ -588,6 +588,7 @@ static inline int userfs_statfs_dispatch(FAR struct userfs_info_s *info,
   /* Dispatch the request */
 
   DEBUGASSERT(info->userops != NULL && info->userops->statfs != NULL);
+  memset(&resp.buf, 0, sizeof(struct statfs));
   resp.ret  = info->userops->statfs(info->volinfo, &resp.buf);
 
   /* Send the response */