selftests: mptcp: add mptcp_info tests

This patch adds the mptcp_info fields tests in endpoint_tests(). Add a
new function chk_mptcp_info() to check the given number of the given
mptcp_info field.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/330


Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>