selftests: forwarding: lib: Avoid trapping soft devices

There are several cases where traffic that would normally be forwarded
in silicon needs to be observed in slow path. That's achieved by
trapping such traffic, and the functions trap_install() and
trap_uninstall() realize that. However, such treatment is obviously
wrong if the device in question is actually a soft device not backed by
an ASIC.

Therefore try to trap if possible, but fall back to inserting a continue
if not.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>