knfsd: allow nfsd READDIR to return 64bit cookies

->readdir passes lofft_t offsets (used as nfs cookies) to
nfs3svc_encode_entry{,_plus}, but when they pass it on to encode_entry it
becomes an 'off_t', which isn't good.

So filesystems that returned 64bit offsets would lose.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Adrian Bunk <bunk@kernel.org>