vhost-net: don't open-code kvfree

Commit 23cc5a99 ("vhost-net: extend device allocation to vmalloc")
added another open-coded version of kvfree (which is available since
v3.15-rc5), nuke it.

Signed-off-by: Romain Francoise <romain@orebokech.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>