NFS: Fix a regression in the referral code

Fix a regression that was introduced by commit
0c2e53f1 (NFS: Remove the unused
"lookupfh()" version of nfs4_proc_lookup()).

In the case where the lookup gets an NFS4ERR_MOVED, we want to return
the result of nfs4_get_referral(). Instead, that value is getting
clobbered by the call to nfs4_handle_exception()...

Reported-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Tested-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>