nfs: nfs4clinet: check the return value of kstrdup()

stable inclusion
from linux-4.19.230
commit 2c9587f72ff4b502c2fd15eb3ccff388eae12d07

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

[ Upstream commit fbd2057e ]

kstrdup() returns NULL when some internal memory errors happen, it is
better to check the return value of it so to catch the memory error in
time.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>