Make all "padding" fields filterable

Change-Id: Ic4e4d9142a9b05ef3b628fac2e8766014c5fccec
Reviewed-on: https://code.wireshark.org/review/4073
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>