fw4: skip not existing netdev names in flowtable device list

fw4: skip not existing netdev names in flowtable device list

In case interface configurations are present which refer to not existing
network devices, such device names might end up in the flowtable list,
leading to `No such file or directory` errors when attempting to load
the resulting ruleset.

Solve this issue by testing for each netdev name whether it refers to
an existing device.

Fixes: e009588 ("fw4: do not add physical devices for soft offload")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>