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

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>