NFSv4: Fix up nfs4_proc_lookup_mountpoint

Currently, we do not check the return value of client = rpc_clone_client(),
nor do we shut down the resulting cloned rpc_clnt in the case where a
NFS4ERR_WRONGSEC has caused nfs4_proc_lookup_common() to replace the
original value of 'client' (causing a memory leak).

Fix both issues and simplify the code by moving the call to
rpc_clone_client() until after nfs4_proc_lookup_common() has
done its business.

Reported-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>