cls/user: implement reset user stats

cls/user: implement reset user stats

This is an implementation of reset user stats, that recalculates the user stats
purely based on the values of bucket entries in user.buckets object. This is
helpful in cases when user stats has been improperly set in case of manual
resharding etc.

Luminous only additions:
use global scope for encode in cls_user_ops

Fixes: http://tracker.ceph.com/issues/23335
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 63f9127dd685890d938a612739320c942a96f910)