osd/OSDMap: don't mapping all pgs each time in calc_pg_upmaps

osd/OSDMap: don't mapping all pgs each time in calc_pg_upmaps

We have a cluster pool with 32768 pgs and 400 osds, it costs 600 seconds when doing upmap
with '--upmap-max 32768 --upmap-deviation 0.01', which is pretty slow.
After adding some debug code, the time mostly spent on pg_to_up_acting_osds, the average
time for one pg_to_up_acting_osds is about 12us, so the whole pool's pg will cost 500ms each
time, we finally have 1429 pgs need to do upmap, so it cost about 600 seconds.
Withi this patch, it only spend 5 seconds to get job done.

Signed-off-by: huangjun <huangjun@xsky.com>