if_up() must be called at splnet or higher.
Second, set the IFF_RUNNING flag on an interface after its
resources (i.e. tunnel source and destination addresses)
have been set. Note that we don't set IFF_UP because it is
if_up()'s job to do that.
PR: kern/27851
Submitted by: Horacio J. PeÓa <horape@compendium.com.ar>