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

[ 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>