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>

fs/nfs/nfs4state.c

@@ -868,7 +868,7 @@ static struct nfs4_lock_state *nfs4_get_lock_state(struct nfs4_state *state, fl_
 	
 	for(;;) {
 		spin_lock(&state->state_lock);
-		lsp = __nfs4_find_lock_state(state, owner, 0);
+		lsp = __nfs4_find_lock_state(state, owner, NULL);
 		if (lsp != NULL)
 			break;
 		if (new != NULL) {