nfs4: fix discover_server_trunking use after free

If clp is new (cl_count = 1) and it matches another client in
nfs4_discover_server_trunking, the nfs_put_client will free clp before
->cl_preserve_clid is set.

Cc: stable@vger.kernel.org # 3.7+
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>