selftests: openvswitch: don't hardcode the drop reason subsys

selftests: openvswitch: don't hardcode the drop reason subsys

WiFi removed one of their subsys entries from drop reasons, in
commit 286e69677065 ("wifi: mac80211: Drop cooked monitor support")
SKB_DROP_REASON_SUBSYS_OPENVSWITCH is now 2 not 3.
The drop reasons are not uAPI, read the correct value
from debug info.

We need to enable vmlinux BTF, otherwise pahole needs
a few GB of memory to decode the enum name.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20250304180615.945945-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>