mptcp: move mptcp_pm_remove_addrs into pm_userspace

mptcp: move mptcp_pm_remove_addrs into pm_userspace

Since mptcp_pm_remove_addrs() is only called from the userspace PM, this
patch moves it into pm_userspace.c.

For this, lookup_subflow_by_saddr() and remove_anno_list_by_saddr()
helpers need to be exported in protocol.h. Also add "mptcp_" prefix for
these helpers.

Here, mptcp_pm_remove_addrs() is not changed to a static function because
it will be used in BPF Path Manager.

This patch doesn't change the behaviour of the code, just refactoring.

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>
Link: https://patch.msgid.link/20241213-net-next-mptcp-pm-misc-cleanup-v1-4-ddb6d00109a8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>