mptcp: add userspace_pm_lookup_addr_by_id helper

Corresponding __lookup_addr_by_id() helper in the in-kernel netlink PM,
this patch adds a new helper mptcp_userspace_pm_lookup_addr_by_id() to
lookup the address entry with the given id on the userspace pm local
address list.

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>