rtnl_neightbl: enhance decoding of struct ndt_stats

rtnl_neightbl: enhance decoding of struct ndt_stats

Add support of kernels that operate with older definition of
struct ndt_stats than the definition used to build strace.

* rtnl_neightbl.c (decode_ndt_stats): Add runtime detection
of struct ndt_stats.ndts_table_fulls field, print the field
when it is available.