libceph: introduce decode{,_new}_pg_temp() and switch to them

Consolidate pg_temp (full map, map<pg_t, vector<u32>>) and new_pg_temp
(inc map, same) decoding logic into a common helper and switch to it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>