mptcp: implement mptcp_userspace_pm_get_addr

This patch implements mptcp_userspace_pm_get_addr() to get an address
from userspace pm address list according the given 'token' and 'id'.
Use nla_get_u32() to get the u32 value of 'token', then pass it to
mptcp_token_get_sock() to get the msk. Pass 'msk' and 'id' to the helper
mptcp_userspace_pm_lookup_addr_by_id() to get the address entry. Put
this entry to userspace using mptcp_pm_nl_put_entry_info().

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>