mptcp: implement mptcp_userspace_pm_dump_addr

This patch implements mptcp_userspace_pm_dump_addr() to dump addresses
from userspace pm address list. Use mptcp_token_get_sock() to get the
msk from the given token, if userspace PM is enabled in it, traverse
each address entry in address list, put every entry to userspace using
mptcp_pm_nl_put_entry_msg().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>