mptcp: make pm_remove_addrs_and_subflows static

[ Upstream commit e38b117d ]

mptcp_pm_remove_addrs_and_subflows() is only used in pm_netlink.c, it's
no longer used in pm_userspace.c any more since the commit 8b1c94da
("mptcp: only send RM_ADDR in nl_cmd_remove"). So this patch changes it
to a static function.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 87b5896f3f78 ("mptcp: pm: fix RM_ADDR ID for the initial subflow")
Signed-off-by: Sasha Levin <sashal@kernel.org>