nfsd4: only require krb5 principal for NFSv4.0 callbacks

In the sessions backchannel case, we don't need a krb5 principal name
for the client; we use the already-created forechannel credentials
instead.

Some cleanup, while we're there: make it clearer which code here is 4.0-
or sessions- specific.

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