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>
Merge pull request #2331 from ceph/wip-9238

mds: fix FP error in ROUND_UP_TO

Reviewed-by: Sage Weil <sage@redhat.com>