nfsd: remove dl_fh field from struct nfs4_delegation

Now that the nfs4_file has a filehandle in it, we no longer need to
keep a per-delegation copy of it. Switch to using the one in the
nfs4_file instead.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>