464xlat: delete SNATed conntracks on interface teardown

Existing conntracks will continue to be SNATed to 192.0.0.1 even after
464xlat interface gets teared down. To prevent this, matching
conntracks must be killed.

Signed-off-by: Alin Nastac <alin.nastac@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]