mptcp: pm: add local parameter for set_flags

This patch updates the interfaces set_flags to reduce repetitive
code, adds a new parameter 'local' for them.

The local address is parsed in public helper mptcp_pm_nl_set_flags_doit(),
then pass it to mptcp_pm_nl_set_flags() and mptcp_userspace_pm_set_flags().

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>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>