zebra: fix detection of interface renames

zebra: fix detection of interface renames

Restore the original logic in netlink_link_change() which works like this:
* once an interface event is detected, lookup the associated interface
  by its name;
* call the set_ifindex() function;
* set_ifindex() will lookup the interface again but now by its ifindex. If
  the lookups by name and ifindex yield to different results, then the
  interface was renamed and set_ifindex() will take care of that.

In the future, zns->if_table will be split into two different data
structures to allow faster lookups by both name and ifindex.

Fixes Issue #397.

Regression introduced by commit 12f6fb9.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>