lwt_bpf: Replace preempt_disable() with migrate_disable()

migrate_disable() is just a wrapper for preempt_disable() in
non-RT kernel. It is safe to replace it, and RT kernel will
benefit.

Note that it is introduced since Feb 2020.

Suggested-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/core/lwt_bpf.c

···
 {
 	int ret;
 
-	/* Preempt disable and BH disable are needed to protect per-cpu
+	/* Migration disable and BH disable are needed to protect per-cpu
 	 * redirect_info between BPF prog and skb_do_redirect().
 	 */
-	preempt_disable();
+	migrate_disable();
 	local_bh_disable();
 	bpf_compute_data_pointers(skb);
 	ret = bpf_prog_run_save_cb(lwt->prog, skb);
···
 	}
 
 	local_bh_enable();
-	preempt_enable();
+	migrate_enable();
 
 	return ret;
 }