libceph: primary_affinity infrastructure

Add primary_affinity infrastructure.  primary_affinity values are
stored in an max_osd-sized array, hanging off ceph_osdmap, similar to
a osd_weight array.

Introduce {get,set}_primary_affinity() helpers, primarily to return
CEPH_OSD_DEFAULT_PRIMARY_AFFINITY when no affinity has been set and to
abstract out osd_primary_affinity array allocation and initialization.

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