struct crush_bucket_tree::num_nodes is u8, so ceph_decode_8_safe()
should be used. -Wconversion catches this, but I guess it went
unnoticed in all the noise it spews. The actual problem (at least for
common crushmaps) isn't the u32 -> u8 truncation though - it's the
advancement by 4 bytes instead of 1 in the crushmap buffer.