NFS: Fix false error return from nfs_callback_up() if ipv6.ko is not available

Clear "ret" if the error return from svc_create_xprt(AF_INET6) was
-EAFNOSUPORT.  Otherwise, callback start-up will succeed, but
nfs_callback_up() will return -EAFNOSUPPORT anyway, and the first
NFSv4 mount attempt after a reboot will fail.

Bug introduced by commit f738f517 in 2.6.30-rc1.
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/callback.c

@@ -127,7 +127,9 @@ int nfs_callback_up(void)
 		nfs_callback_tcpport6 = ret;
 		dprintk("NFS: Callback listener port = %u (af %u)\n",
 				nfs_callback_tcpport6, PF_INET6);
-	} else if (ret != -EAFNOSUPPORT)
+	} else if (ret == -EAFNOSUPPORT)
+		ret = 0;
+	else
 		goto out_err;
 #endif	/* defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) */