selftests: mptcp: netlink: add change_address helper

The output formats of 'ip mptcp' commands are much different from that
of 'pm_nl_ctl' commands.

A new 'change_address' helper is added here, to change the flag of an
address. This is a bit similar to mptcp_join.sh's pm_nl_change_endpoint().

Usage:
	Address ID - pm_nl_change_endpoint $ns id $id $flags
	IP address - change_address $ns $addr $flags

Use this new helper in pm_netlink.sh to replace all 'pm_nl_ctl set'
commands.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>