mptcp: add port support for setting flags

mptcp: add port support for setting flags

This patch updated the port keyword check for the setting flags, allow
to use the port keyword with the non-signal flags. Don't allow to use
the port keyword with the id number.

With this patch, we can use setting flags in two forms, using the address
and port number directly or the id number of the address:

 ip mptcp endpoint change id 1 fullmesh
 ip mptcp endpoint change 10.0.2.1 fullmesh
 ip mptcp endpoint change 10.0.2.1 port 10100 fullmesh

Acked-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: David Ahern <dsahern@kernel.org>