tcp: tcp_check_req() can be called from process context

This is a follow up of commit 0a375c8 ("tcp: tcp_rtx_synack()
can be called from process context").

Frederick Lawler reported another "__this_cpu_add() in preemptible"
warning caused by the same reason.

In my former patch I took care of tcp_rtx_synack()
but forgot that tcp_check_req() also contained some SNMP updates.

Note that some parts of tcp_check_req() always run in BH context,
I added a comment to clarify this.

Fixes: 8336886 ("tcp: TCP Fast Open Server - support TFO listeners")
Link: https://lore.kernel.org/netdev/8cd33923-a21d-397c-e46b-2a068c287b03@cloudflare.com/T/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Frederick Lawler <fred@cloudflare.com>
Tested-by: Frederick Lawler <fred@cloudflare.com>
Link: https://lore.kernel.org/r/20230227083336.4153089-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_minisocks.c

@@ -597,6 +597,9 @@ EXPORT_SYMBOL(tcp_create_openreq_child);
  * validation and inside tcp_v4_reqsk_send_ack(). Can we do better?
  *
  * We don't need to initialize tmp_opt.sack_ok as we don't use the results
+ *
+ * Note: If @fastopen is true, this can be called from process context.
+ *       Otherwise, this is from BH context.
  */
 
 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
@@ -748,7 +751,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 					  &tcp_rsk(req)->last_oow_ack_time))
 			req->rsk_ops->send_ack(sk, skb, req);
 		if (paws_reject)
-			__NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
+			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
 		return NULL;
 	}
 
@@ -767,7 +770,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	 *	   "fourth, check the SYN bit"
 	 */
 	if (flg & (TCP_FLAG_RST|TCP_FLAG_SYN)) {
-		__TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
+		TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
 		goto embryonic_reset;
 	}