libceph: replace BUG_ON with bounds check for map->max_osd

In the Linux kernel, the following vulnerability has been resolved: libceph: replace BUG_ON with bounds check for map->max_osd OSD indexes come from untrusted network packets. Boundary checks are added to validate these against map->max_osd. [ idryomov: drop BUG_ON in ceph_get_primary_affinity(), minor cosmetic edits ]