selftests: bpf: log direct file writes

Recent changes to netdevsim moved creating and destroying
devices from netlink to sysfs. The sysfs writes have been
implemented as direct writes, without shelling out. This
is faster, but leaves no trace in the logs. Add explicit
logs to make debugging possible.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>