nfsd: Detect unhashed stids in nfsd4_verify_open_stid()

The state of the stid is guaranteed by 2 locks:
- The nfs4_client 'cl_lock' spinlock
- The nfs4_ol_stateid 'st_mutex' mutex

so it is quite possible for the stid to be unhashed after lookup,
but before calling nfsd4_lock_ol_stateid(). So we do need to check
for a zero value for 'sc_type' in nfsd4_verify_open_stid().
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>
Tested-by: NChecuk Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org
Fixes: 659aefb6 "nfsd: Ensure we don't recognise lock stateids..."
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -3590,6 +3590,7 @@ nfsd4_verify_open_stid(struct nfs4_stid *s)
 	switch (s->sc_type) {
 	default:
 		break;
+	case 0:
 	case NFS4_CLOSED_STID:
 	case NFS4_CLOSED_DELEG_STID:
 		ret = nfserr_bad_stateid;