mptcp: introduce token KUNIT self-tests

Unit tests for the internal MPTCP token APIs, using KUNIT

v1 -> v2:
 - use the correct RCU annotation when initializing icsk ulp
 - fix a few checkpatch issues

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>