NFSv4: Fix warning for using 0 as NULL

Fixes the following sparse warning:

fs/nfs/nfs4state.c:862:60: warning: Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>