selftests/bpf: Rename reference_tracking BPF programs

BPF programs for reference_tracking selftest use "fail_" prefix to notify that
they are expected to fail. This is really confusing and inconvenient when
trying to grep through test_progs output to find *actually* failed tests. So
rename the prefix from "fail_" to "err_".

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210805230734.437914-1-andrii@kernel.org