bpf: Remove preempt_disable in bpf_try_get_buffers

Now BPF program will run with migration disabled, so it is safe
to access this_cpu_inc_return(bpf_bprintf_nest_level).

Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf")
Signed-off-by: Tao Chen <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/helpers.c

···
 {
 	int nest_level;
 
-	preempt_disable();
 	nest_level = this_cpu_inc_return(bpf_bprintf_nest_level);
 	if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) {
 		this_cpu_dec(bpf_bprintf_nest_level);
-		preempt_enable();
 		return -EBUSY;
 	}
 	*bufs = this_cpu_ptr(&bpf_bprintf_bufs[nest_level - 1]);
···
 	if (WARN_ON_ONCE(this_cpu_read(bpf_bprintf_nest_level) == 0))
 		return;
 	this_cpu_dec(bpf_bprintf_nest_level);
-	preempt_enable();
 }
 
 void bpf_bprintf_cleanup(struct bpf_bprintf_data *data)