Trying to run iptables/nftables on Synology pauses for minutes with
lots of errors and ultimately does nothing as it's not used and we
lack permissions.
This fixes a regression from db760d0bacd351a77f4 (#11601) that landed
between Synology testing on unstable 1.63.110 and 1.64.0 being cut.
Fixes#11737
Change-Id: Iaf9563363b8e45319a9b6fe94c8d5ffaecc9ccef
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>