crushtool: make --reweight re-sum choose_args weight-sets too

crushtool: make --reweight re-sum choose_args weight-sets too

This ensures that the weights add us for each weight-set (and each
position).  Note that since we don't have anything that actually
creates positional weight-sets, the behavior here might not be what we
want in the end, but for the compat weight-sets (no position), we *do*
keep the weights as a properly summing tree.

Signed-off-by: Sage Weil <sage@redhat.com>