selftests/bpf: fix netdevsim trap_flow_action_cookie read

When read netdevsim trap_flow_action_cookie, we need to init it first,
or we will get "Invalid argument" error.

Fixes: d3cbb907 ("netdevsim: add ACL trap reporting cookie as a metadata")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>