firewall: properly handle REJECT rules

firewall: properly handle REJECT rules

Currently we generated DROP statements for all rules involving REJECT.
We only need to generate DROP when in the postrouting chain of tables
with type bridge, since REJECT is disallowed there. Otherwise we jump
into the do-reject chain which properly handles rejects for different
protocol types.

Reported-By: Stefan Sterz <s.sterz@proxmox.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>