fix #4175: ignore non-filter ebtables tables

fix #4175: ignore non-filter ebtables tables

we only ever add rules to the filter table, without this we'd add all
rules from other tables (which might have been manually filled by the
admin) to the filter table as well - adding another copy on every
iteration of the firewall update cycle!

note that ebtables-restore seems to flush tables contained in its input,
but leave those alone which are not referenced at all.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>