The old NFS client will crash due to the reply being m_freem()'d

twice if the server bogusly returns an error with the NFSERR_RETERR
bit (bit 31) set. No actual NFS error has this bit set, but it seems
that amd will sometimes do this. This patch makes sure the NFSERR_RETERR
bit is cleared to avoid a crash.

PR:		kern/153847
MFC after:	2 weeks

sys/nfsclient/nfs_krpc.c

@@ -540,6 +540,11 @@ nfs_request(struct vnode *vp, struct mbuf *mreq, int procnum,
 				    hz);
 			goto tryagain;
 		}
+		/*
+		 * Make sure NFSERR_RETERR isn't bogusly set by a server
+		 * such as amd. (No actual NFS error has bit 31 set.)
+		 */
+		error &= ~NFSERR_RETERR;
 
 		/*
 		 * If the File Handle was stale, invalidate the lookup