tcp: tcp_check_req() can be called from process context

stable inclusion
from stable-v5.10.173
commit ac73d8f6a64a9be3ab1c9e8d9ed66d81aebc82f5
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7X0QU

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ac73d8f6a64a9be3ab1c9e8d9ed66d81aebc82f5

--------------------------------

[ Upstream commit 580f98cc ]

This is a follow up of commit 0a375c82 ("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: 8336886f ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>