MFC 258591

MFC 258591
In vnet_route_uninit(), free some memory that is allocated in vnet_route_init().

To reproduce the problem:
  (1)  Take a GENERIC kernel config, and add options for: VIMAGE, WITNESS,
       INVARIANTS.
  (2)  Run this command in a loop:
       jail -l -u root -c path=/ name=foo persist vnet && jexec foo ifconfig lo0 127.0.0.1/8 && jail -r foo

       see: http://lists.freebsd.org/pipermail/freebsd-current/2010-November/021280.html
            http://lists.freebsd.org/pipermail/freebsd-current/2010-November/021291.html

This doesn't eliminate all the "Freed UMA keg was not empty" warning messages
on the console, but it helps.

Approved by: re (gjb)

git-svn-id: svn://svn.freebsd.org/base/stable/10@258913 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f