crush: fix distortion of straw scalers by 0-weight items

crush: fix distortion of straw scalers by 0-weight items

The presence of a 0-weight item in a straw bucket should have no effect
on the placement of other items.  Add a test validating that and fix
crush_calc_straw() to fix the distortion.

Note that this effects the *precalculation* of the straw bucket inputs and
does not effect the actually mapping process given a compiled or encoded
CRUSH map, and only when straw_calc_version == 1 (i.e., the admin opted in
to the new behavior).

Backport: giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 85498bc8f62ca56506b33f3c5ec4fc4b111ed73d)