Este commit resolverá esse problema.
commit e890db0104826742818cbfb8fdb3000a38a9b97c
NFSv4: Corrija a opção de montagem de sincronização para montagens nfs4
diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index 71fdc0d..f6db66d 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -2478,6 +2478,10 @@ struct dentry *nfs_fs_mount_common(struct nfs_server *server,
if (server->flags & NFS_MOUNT_NOAC)
sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+ if (mount_info->cloned != NULL && mount_info->cloned->sb != NULL)
+ if (mount_info->cloned->sb->s_flags & MS_SYNCHRONOUS)
+ sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
/* Get a superblock - note that we may end up sharing one that already exists */
s = sget(nfs_mod->nfs_fs, compare_super, nfs_set_super, flags, &sb_mntdata);
if (IS_ERR(s)) {