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