bpf: Disable preemption in bpf_perf_event_output

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

bpf: Disable preemption in bpf_perf_event_output

The nesting protection in bpf_perf_event_output relies on disabled
preemption, which is guaranteed for kprobes and tracepoints.

However bpf_perf_event_output can be also called from uprobes context
through bpf_prog_run_array_sleepable function which disables migration,
but keeps preemption enabled.

This can cause task to be preempted by another one inside the nesting
protection and lead eventually to two tasks using same perf_sample_data
buffer and cause crashes like:

kernel tried to execute NX-protected page - exploit attempt? (uid: 0)
BUG: unable to handle page fault for address: ffffffff82be3eea
...
Call Trace:
? __die+0x1f/0x70
? page_fault_oops+0x176/0x4d0
? exc_page_fault+0x132/0x230
? asm_exc_page_fault+0x22/0x30
? perf_output_sample+0x12b/0x910
? perf_event_output+0xd0/0x1d0
? bpf_perf_event_output+0x162/0x1d0
? bpf_prog_c6271286d9a4c938_krava1+0x76/0x87
? __uprobe_perf_func+0x12b/0x540
? uprobe_dispatcher+0x2c4/0x430
? uprobe_notify_resume+0x2da/0xce0
? atomic_notifier_call_chain+0x7b/0x110
? exit_to_user_mode_prepare+0x13e/0x290
? irqentry_exit_to_user_mode+0x5/0x30
? asm_exc_int3+0x35/0x40

Fixing this by disabling preemption in bpf_perf_event_output.