bpf,perf: Fix invalid prog_array access in perf_event_detach_bpf_prog

In the Linux kernel, the following vulnerability has been resolved:

bpf,perf: Fix invalid prog_array access in perf_event_detach_bpf_prog

Syzbot reported [1] crash that happens for following tracing scenario:

- create tracepoint perf event with attr.inherit=1, attach it to the

process and set bpf program to it

- attached process forks -> chid creates inherited event

the new child event shares the parent's bpf program and tp_event

(hence prog_array) which is global for tracepoint

- exit both process and its child -> release both events

- first perf_event_detach_bpf_prog call will release tp_event->prog_array

and second perf_event_detach_bpf_prog will crash, because

tp_event->prog_array is NULL

The fix makes sure the perf_event_detach_bpf_prog checks prog_array

is valid before it tries to remove the bpf program from it.

[1] https://lore.kernel.org/bpf/Z1MR6dCIKajNS6nU@krava/T/#m91dbf0688221ec7a7fc95e896a7ef9ff93b0b8ad