selftests: mptcp: add the net device name testcase

This patch added a new testcase for setting the net device name. In it,
pass the net device name to pm_nl_ctl to set the ifindex field of struct
mptcp_pm_addr_entry.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>