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

stable inclusion
from stable-v5.10.101
commit c5619c510f04d7ceaa0136cd5fab9960c3d23b65
bugzilla: https://gitee.com/openeuler/kernel/issues/I5669Z

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c5619c510f04d7ceaa0136cd5fab9960c3d23b65



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[ 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: Yu Liao <liaoyu15@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>