findif.sh: fix loopback handling

tools/ocft/IPaddr2 fails the loopback test because of the missing
table local parameter:

$ ip -o -f inet route list match 127.0.0.3 scope host

$ ip -o -f inet route list match 127.0.0.3 table local scope host
local 127.0.0.0/8 dev lo proto kernel src 127.0.0.1

Also rename the function because it is called only in for the special
loopback address case.