NFSv4.1: Enable state protection

Use the EXCHGID4_FLAG_BIND_PRINC_STATEID exchange_id flag to enable
stateid protection. This means that if we create a stateid using a
particular principal, then we must use the same principal if we
want to change that state.
IOW: if we OPEN a file using a particular credential, then we have
to use the same credential in subsequent OPEN_DOWNGRADE, CLOSE,
or DELEGRETURN operations that use that stateid.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -5528,7 +5528,8 @@ int nfs4_proc_exchange_id(struct nfs_client *clp, struct rpc_cred *cred)
 	struct nfs41_exchange_id_args args = {
 		.verifier = &verifier,
 		.client = clp,
-		.flags = EXCHGID4_FLAG_SUPP_MOVED_REFER,
+		.flags = EXCHGID4_FLAG_SUPP_MOVED_REFER |
+			EXCHGID4_FLAG_BIND_PRINC_STATEID,
 	};
 	struct nfs41_exchange_id_res res = {
 		0