NFS: Use kmemdup_nul() in nfs_readdir_make_qstr()

mainline inclusion
from mainline-v5.6-rc1
commit 3803d672
category: bugfix
bugzilla: NA
CVE: NA

--------------------------------

The directory strings stored in the readdir cache may be used with
printk(), so it is better to ensure they are nul-terminated.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>