mptcp: pm: kernel: flush: do not reset ADD_ADDR limit

A flush of the MPTCP endpoints should not affect the MPTCP limits. In
other words, 'ip mptcp endpoint flush' should not change 'ip mptcp
limits'.

But it was the case: the MPTCP_PM_ATTR_RCV_ADD_ADDRS (add_addr_accepted)
limit was reset by accident. Removing the reset of this counter during a
flush fixes this issue.

Fixes: 01cacb0 ("mptcp: add netlink-based PM")
Cc: stable@vger.kernel.org
Reported-by: Thomas Dreibholz <dreibh@simula.no>
Closes: multipath-tcp/mptcp_net-next#579
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250815-net-mptcp-misc-fixes-6-17-rc2-v1-2-521fe9957892@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

net/mptcp/pm_kernel.c

@@ -1085,7 +1085,6 @@ static void __flush_addrs(struct list_head *list)
 static void __reset_counters(struct pm_nl_pernet *pernet)
 {
 	WRITE_ONCE(pernet->add_addr_signal_max, 0);
-	WRITE_ONCE(pernet->add_addr_accept_max, 0);
 	WRITE_ONCE(pernet->local_addr_max, 0);
 	pernet->addrs = 0;
 }