mptcp: pm: add mptcp_pm_genl_fill_addr helper

mptcp: pm: add mptcp_pm_genl_fill_addr helper

To save some redundant code in dump_addr() interfaces of both the
netlink PM and userspace PM, the code that calls netlink message
helpers (genlmsg_put/cancel/end) and mptcp_nl_fill_addr() is wrapped
into a new helper mptcp_pm_genl_fill_addr().

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/20250221-net-next-mptcp-pm-misc-cleanup-3-v1-4-2b70ab1cee79@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>