libbpf: Cleanup the legacy kprobe_event on failed add/attach_event()

Before the 0bc11ed5 commit ("kprobes: Allow kprobes coexist with
livepatch"), in a scenario where livepatch and kprobe coexist on the
same function entry, the creation of kprobe_event using
add_kprobe_event_legacy() will be successful, at the same time as a
trace event (e.g. /debugfs/tracing/events/kprobe/XXX) will exist, but
perf_event_open() will return an error because both livepatch and kprobe
use FTRACE_OPS_FL_IPMODIFY. As follows:

1) add a livepatch

$ insmod livepatch-XXX.ko

2) add a kprobe using tracefs API (i.e. add_kprobe_event_legacy)

$ echo 'p:mykprobe XXX' > /sys/kernel/debug/tracing/kprobe_events

3) enable this kprobe (i.e. sys_perf_event_open)

This will return an error, -EBUSY.

On Andrii Nakryiko's comment, few error paths in
bpf_program__attach_kprobe_opts() that should need to call
remove_kprobe_event_legacy().

With this patch, whenever an error is returned after
add_kprobe_event_legacy() or bpf_program__attach_perf_event_opts(), this
ensures that the created kprobe_event is cleaned.

Signed-off-by: Chuang Wang <nashuiliang@gmail.com>
Signed-off-by: Jingren Zhou <zhoujingren@didiglobal.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220629151848.65587-2-nashuiliang@gmail.com