mptcp: update set_flags interfaces

This patch updates set_flags interfaces, make it more similar to the
interfaces of dump_addr and get_addr:

 mptcp_pm_set_flags(struct sk_buff *skb, struct genl_info *info)
 mptcp_pm_nl_set_flags(struct sk_buff *skb, struct genl_info *info)
 mptcp_userspace_pm_set_flags(struct sk_buff *skb, struct genl_info *info)

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://lore.kernel.org/r/20240305-upstream-net-next-20240304-mptcp-misc-cleanup-v1-2-c436ba5e569b@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>