NFSv4: handle nfs4_get_referral failure

nfs4_proc_lookup_common is supposed to return a posix error, we have to
handle any error returned that isn't errno

Reported-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>