nfsd4: eliminate struct nfs4_cb_recall

The nfs4_cb_recall struct is used only in nfs4_delegation, so its
pointer to the containing delegation is unnecessary--we could just use
container_of().

But there's no real reason to have this a separate struct at all--just
move these fields to nfs4_delegation.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>