rtadvd(8): Don't use-after-free

This whole block of code as committed fully formed in r224144. I'm not really
sure what the intent was, but it seems plausible that !persist ifis could need
other member cleanup. Don't free the object until after we've finished
cleaning its members.

Reported by: Coverity
CID: 1006079
Sponsored by: EMC / Isilon Storage Division