nfsd4: simplify process_lookup1

This STALE_CLIENTID check is redundant with the one in
lookup_clientid().

There's a difference in behavior is in case of memory allocation
failure, which I think isn't a big deal.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>