osd: calculate initial PG mapping from PG's osdmap

The initial values of up/acting need to be based on the PG's osdmap, not
the OSD's latest.  This can cause various confusion in
pg_interval_t::check_new_interval() when calling OSDMap methods due to the
up/acting OSDs not existing yet (for example).

Fixes: #3879
Reported-by: Jens Kristian S?gaard <jens@mermaidconsulting.dk>
Tested-by: Jens Kristian S?gaard <jens@mermaidconsulting.dk>
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>