Assert that the mbuf in the chain has sane length. Proper place for

this check is somewhere in the network code, but this assertion
already proven to be useful in catching what seems to be driver bugs
causing NFS scrambling random memory.

Discussed with:	rmacklem
MFC after:	1 week

sys/fs/nfs/nfs_commonsubs.c

@@ -218,6 +218,7 @@ nfsm_mbufuio(struct nfsrv_descript *nd, struct uio *uiop, int siz)
 				}
 				mbufcp = NFSMTOD(mp, caddr_t);
 				len = mbuf_len(mp);
+				KASSERT(len > 0, ("len %d", len));
 			}
 			xfer = (left > len) ? len : left;
 #ifdef notdef