nfsd4: shut down more of delegation earlier

Once we've unhashed the delegation, it's only hanging around for the
benefit of an oustanding recall, which only needs the encoded
filehandle, stateid, and dl_retries counter.  No point keeping the file
around any longer, or keeping it hashed.

This also fixes a race: calls to idr_remove should really be serialized
by the caller, but the nfs4_put_delegation call from the callback code
isn't taking the state lock.

(Better might be to cancel the callback before destroying the
delegation, and remove any need for reference counting--but I don't see
an easy way to cancel an rpc call.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>