Fix inversion between v4 and v6 in ValidateAttribute
Fix inversion between v4 and v6 in ValidateAttribute

This fixes a trivial inversion of v4/v6 calls
in ValidateAttribute.

Bug-Url: #991

ramrunner/gobgp,a16/gobgp,fujita/gobgp,tamihiro/gobgp,a16/gobgp,fujita/gobgp,j-kato/gobgp,ramrunner/gobgp,a16/gobgp,tamihiro/gobgp,osrg/gobgp,ishidawataru/gobgp,a16/gobgp,ishidawataru/gobgp,fujita/gobgp,osrg/gobgp,Dipsingh/gobgp,j-kato/gobgp,tamihiro/gobgp,ramrunner/gobgp,fujita/gobgp,a16/gobgp,tamihiro/gobgp,Dipsingh/gobgp,a16/gobgp,osrg/gobgp,j-kato/gobgp,ishidawataru/gobgp,a16/gobgp,osrg/gobgp,Dipsingh/gobgp