selftests: mptcp: fix typo in mptcp_connect usage

in mptcp_connect, 's' selects IPPROTO_MPTCP / IPPROTO_TCP as the value of
'protocol' in socket(), and 'm' switches between different send / receive
modes. Fix die_usage(): swap 'm' and 's' and add missing 'sendfile' mode.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>