in_rtalloc1(9) returns a locked route, so make sure that we use
RTFREE_LOCKED() here. This macro makes sure the reference count
on the route is being managed properly. This elimates another
case which results in the following message being printed to the
console:

rtfree: 0xc841ee88 has 1 refs

Reviewed by: bz
MFC after: 2 weeks