nfsd/blocklayout: Make sure calculate signature/designator length aligned

These values are all multiples of 4 already, so there's no change in
behavior from this patch.  But perhaps this will prevent mistakes in the
future.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>