nfsd: Move the delegation reference counter into the struct nfs4_stid

We will want to add reference counting to the lock stateid and open
stateids too in later patches.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>