selftests: mptcp: set fullmesh flag in pm_nl_ctl

This patch added the fullmesh flag setting and clearing support in
pm_nl_ctl:

 # pm_nl_ctl set ip flags fullmesh
 # pm_nl_ctl set ip flags nofullmesh

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>