[th/platform-struct-packing] reorder some fields in NMPlatformIP(Address,Route) structs a use flexible array members

try to address a coverity warning here : https://cov01.lab.eng.brq.redhat.com/covscanhub/waiving/93468/255647/#defects

Also, the use of our _COMMON macro is a bit ugly. Try to make it more correct, add code comments and pack the fields tighter.