libceph: fix pg_temp mapping calculation

We need to apply the modulo pg_num calculation before looking up a pgid in
the pg_temp mapping rbtree.  This fixes pg_temp mappings, and fixes
(some) misdirected requests that result in messages like

[WRN] client4104 10.0.1.219:0/275025290 misdirected client4104.1:129 0.1 to osd0 not [1,0] in e11/11

on the server and stall make the client block without getting a reply (at
least until the pg_temp mapping goes way, but that can take a long long
time).

Reorder calc_pg_raw() a bit to make more sense.

Signed-off-by: Sage Weil <sage@newdream.net>