osd: prefer acting osds in calc_acting()

osd: prefer acting osds in calc_acting()

We currently prefer up osds, and then pull sequentially from peer_info
(strays we know about at the time).  This adds an additional preference
for the current acting, which means we can avoid changes to acting when
they are largely useless.

In particular, I observed that we chose [5,3] and later (when recovery
completed) chose [5,1] because we had since heard about an eligible stray
on 1.  That switch was basically a waste...

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>