Consolidate loopback address tests

Consolidate loopback address tests

Many different places were testing if an address is a loopback
address. Consolidate these into one function to make it easier to
alter.

Change-Id: I9c4465f3bdc4e840c83d69456504caaac1c544e7
Reviewed-on: http://gerrit.openafs.org/2375
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>