nfsd4: hash deleg stateid only on successful nfs4_set_delegation

We don't want the stateid to be found in the hash table before the delegation
is granted.

Currently this is protected by the client_mutex, but we want to break that
up and this is a necessary step toward that goal.

Signed-off-by: Benny Halevy <bhalevy@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>