selftests: mptcp: add wrapper for setting flags

This patch implemented a new function named pm_nl_set_endpoint(), wrapped
the PM netlink commands 'ip mptcp endpoint change flags' and 'pm_nl_ctl
set flags' in it, and used a new argument 'ip_mptcp' to choose which one
to use to set the flags of the PM endpoint.

'ip mptcp' used the ID number argument to find out the address to change
flags, while 'pm_nl_ctl' used the address and port number arguments. So
we need to parse the address ID from the PM dump output as well as the
address and port number.

Used this wrapper in do_transfer() instead of using the pm_nl_ctl command
directly.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>