nfsd: eliminate "to_delegation" define

We now have cb_to_delegation and to_delegation, which do the same thing
and are defined separately in different .c files. Move the
cb_to_delegation definition into a header file and eliminate the
redundant to_delegation definition.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>