defaults: robustify flow table detection.

defaults: robustify flow table detection.

The flow table detection fails if the respective target module is
built into the kernel, since it's looking for the module itself.

Create a generic helper and instead check for existence of the
FLOWOFFLOAD target in /proc/net/ip_tables_targets.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>
[slightly reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>