nfsd: Move the open owner hash table into struct nfs4_client

Preparation for removing the client_mutex.

Convert the open owner hash table into a per-client table and protect it
using the nfs4_client->cl_lock spin lock.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>