ipaddress: Improve print_linkinfo()

ipaddress: Improve print_linkinfo()

There are few places to improve:

  1) return -1 when entry is filtered instead of zero, which means
     accept entry: ipaddress_list_flush_or_save() the only user of this

  2) use ll_idx_n2a() as last resort to translate name to index for
     "should never happen" cases when cache shouldn't be considered

  3) replace open coded access to IFLA_IFNAME attribute data by
     RTA_DATA() with rta_getattr_str()

  4) simplify ifname printing since name is never NULL, thanks to (2).

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>