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: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>