mds: fix FP error in ROUND_UP_TO

Explicitly handle case where denominator is 0, instead of
passing into ROUND_UP_TO.

Regression from 9449520b121fc6ce0c64948386d4ff77f46f4f5f

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit bf3e4835dabc057982def1b5c9a6499c04ac5312)