net: Do not leak ifa in ip_output

This is backport from freebsd:

   commit ab0c56942296d702f3fb3e9418d3cde78711097b
   Author: glebius <glebius@FreeBSD.org>
   Date:   Wed Jul 9 07:48:05 2014 +0000

       In several cases in ip_output() we obtain reference on ifa. Do not
       leak it.

Signed-off-by: Asias He <asias@cloudius-systems.com>