erasure-code: ROUND_UP_TO accepts any rounding factor

The ROUND_UP_TO function was limited to rounding factors that are powers
of two. This saves a modulo but it is not used where it would make a
difference. The implementation is changed so it is generic.

http://tracker.ceph.com/issues/9209 Fixes: #9209

Signed-off-by: Loic Dachary <loic-201408@dachary.org>