NFSv4.1: Fix an Oops in nfs41_walk_client_list

If we start state recovery on a client that failed to initialise correctly,
then we are very likely to Oops.

Reported-by: "Mkrtchyan, Tigran" <tigran.mkrtchyan@desy.de>
Link: http://lkml.kernel.org/r/130621862.279655.1421851650684.JavaMail.zimbra@desy.de


Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>