nfsd: fix error value on allocation failure in nfsd4_decode_test_stateid()

PTR_ERR(NULL) is going to be 0...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/nfsd/nfs4xdr.c

@@ -1392,7 +1392,7 @@ nfsd4_decode_test_stateid(struct nfsd4_compoundargs *argp, struct nfsd4_test_sta
 	for (i = 0; i < test_stateid->ts_num_ids; i++) {
 		stateid = kmalloc(sizeof(struct nfsd4_test_stateid_id), GFP_KERNEL);
 		if (!stateid) {
-			status = PTR_ERR(stateid);
+			status = nfserrno(-ENOMEM);
 			goto out;
 		}