lockd: Store the lockd client credential in struct nlm_host

When we create a new lockd client, we want to be able to pass the
correct credential of the process that created the struct nlm_host.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>